D..
I think it's probably much worse to lose weight in the 2nd and 3rd trimester. The thing about it is that a baby is kind of like a parasite (sorry!) It will take from the mother to protect itself. The one to worry about is YOU. You need plenty of calcium/vitamin D and magnesium because the baby will pull it out of your bones. That makes you really susceptible.
Once you get to 28 weeks or so, your doctor will REALLY want you to gain weight, at least 1 pound a week.
If I were you, I'd make each and every thing that you put in your mouth COUNT. Look at a pregnancy book and see a menu. You can also make protein and fruit milkshakes to help. Remember that protein is important. It feeds the baby's brain.
I would step back on the walking, to be honest. The threatened miscarriage is reason enough to limit your exercise. Bed rest is serious business.
Another thing I want to tell you, having been on bed rest myself, is that you need to make sure that you drink the amount of water the doctor says to drink. Every day. Have it measured out in the morning and sip on it all day. The uterus is a smooth muscle tissue, D.. If it becomes dehydrated, it starts to contract. AND, a full bladder pushes against the uterus and THAT can make you contract, too. So, what you need to do is drink and pee, drink and pee. A hydrated uterus and an empty bladder is very important. Don't hold your bladder. It's really important not to.
I hope you've been taking pregnancy vitamins since you got pregnant. They are very important.
As you get bigger, try to lay and sleep on your left side. There is a large blood vessel that runs through your spine and the weight of your uterus/baby cuts some of the blood flow off, which is why you shouldn't lay on your back. Research has determined that the left side is optimal (I don't remember why).
Don't overdo. Your hormones are really different right now and when you are tired, REST. Sleep as much as you can. A clean house can wait.
Good luck. It's not easy being pregnant, for many of us.