Your daughter may have a sensory processing disorder which is what I think the book to which you refer is talking about. But I suspect that given all of the things that have happened to her in the past year she is dealing with an overwhelming amount of emotion. She is angry because she is no longer well. She is grieving the loss of good health. And most importantly she is scared, scared, scared because she isn't old enough to understand most of what is happening.
I suggest that you provide her with lots of loving attention while still being consistent with expecting good behavior. Look at discipline as a way of teaching her how to behave rather than as a way to punish her so that she will want to behave.
Being strict doesn't teach. It just puts on more pressure to behave even tho the child would behave if they knew how to handle their emotions.
Tantrums are caused by frustration. Frustration at not being able to be understood. Frustration at not being able to do something. In her case the something is to be a normal healthy child.
There is no such thing as too much empathy. Always be empathic even during the tantrum. What worked for my granddaughter was to sit down in the same room with her while she kicked and screamed. I didn't say anything to her. I just sat and waited. When she became calm, I at first, asked her if she wanted a hug. Before long, once she was calmer she would crawl over to me and I held her for as long as she wanted to be held. Sometimes I would rock her and say something like, (life) is really tough sometimes." or "I know you want to do such and such. It's really difficult not being able to do it." or even, "I'm so sorry that my saying, no, made you angry."
I watch for melt down situations and tried to head them off by giving her words to express how she was feeling. With my daughter, I learned to word my statements as a possibility rather than a fact. for example: "I wonder if you're feeling /frustrated right now?" If her answer was no but she was still calm enough that suggesting another feeling might help, I'd say "angry?" At 6 she may know how she's feeling even tho she couldn't put that feeling into words and I could guess wrong. That is why I worded my statement as a question.
I wouldn't ask her how she was feeling. That might feel too intrusive to her. She also might not yet know how she's feeling. Your question may help her get away from acting out and into dealing with her feelings with your help. Keep in mind how she might be feeling. Focus on her feelings. Let go of your own feelings at that moment. You can go back and address them later once she's OK. And you'll have less frustration because you'll have headed off a tantrum.
Also keep in mind that children are more easily upset and get out of control when they're tired and/or hungry. Give her a snack if you think she may be hungry. Give her quiet time before she gets upset if she's tired.
Since your daughter has health issues she will be more sensitive not only to being tired and hungry but also to myriad other things. A calm, consistent environment is important to staving off tantrums and outbursts. Arrange a room or a corner which is all hers to which she can go before her world overpowers her. Help her to recognize when she needs to go to this quiet place. Have books, music, art materials available for her in this place so that she can divert herself.
Even, when you're away from home you can divert her attention before she's overwhelmed by her feelings. My granddaughter likes to draw and my grandson to write letters of the alphabet. I always have paper and pencil with me when we're out together. My grandson likes Hot Wheels so I keep 2-3 of those around. My granddaughter likes Littlest Pet Shops and so I have some of those. Giving them those things to do only works if I'm sensitive to the early signals of a meltdown.
I've found that once I notice either of them whining I can often stop a meltdown before it starts by giving them personal and warm attention. Sometimes getting them interested in something else works. Sometimes an offer of a hug or holding them on my lap works.
Sometimes they just have to have that meltdown. Often they just cry and only express their anger with words. If I try to talk them out of the meltdown it escalates.
I wouldn't be too concerned over your daughter saying no body loves me if most of the time she feels loved. My granddaughter also sometimes says "no one loves me." I don't try to convince her that I love her or that she's loved. At the moment she feels unloved. I hold he, if she'll let me and repeat, "you're feeling unloved even tho I love you" She'll usually nod and keep on crying but stop saying the words. Feelings are just as real as fact but feelings and fact are not always the same thing.
Sometimes she's angry and yells, "you don't love me! You don't understand me!" I just listen and wait for her to calm down. Often she asks for a hug but if she doesn't I ask her if she wants a hug. Sometimes she doesn't but later she'll often give me a hug later.
You are not responsible for how she feels. She knows you love her. Her feeling unloved is related to many things mostly likely unrelated to how much you show her love. I've found that when I keep myself aware that I'm not supposed to make someone feel happy that I'm better able to accept how they're feeling and be empathic. It also helps to be aware that no one is always happy and that it's OK to feel angry, sad, frustrated, unhappy etc. What is important is to accept those feelings as being OK in ourselves and in others.
What is important is to allow your daughter to express her feelings any way that they come out as long as she isn't physically hurting herself or someone else. At 6 she also needs to learn words as a way of expressing her feelings. Give her the words before she has a meltdown. Talk with her and most importantly listen to her while having a conversation about what is happening to her and how she feels about it during calm times.
When I consider all of the negative things that have happened to her I suggest that professional counseling with a child therapist on a weekly basis might be necessary for her to regain a sense of well being. You could look for one that has experience dealing with sensory disorder issues in case that is a part of her situation.