I think it is both: hormones and mother's instinct.
You definitely are still riding the hormone wave at the 3 month mark, especially if you are breastfeeding. I think mine got better around the 4 month mark, but I didn't breastfeed.
You may notice as your daughter becomes better able to "protect" herself: sitting up, rolling over, swatting something away, manipulating things with her hands, etc.. that you are less on the attack at all times. I also found that as my daughter spent more time with other kids, I became less crazed. I have a group of moms that I am close with and we have regular playdates. Seeing all the kids interact and how the moms "do their thing" helped me put my anxieties into perspective. I also got to see how my daughter handled "situations" and what a tough cookie she could be!
I know what you mean about the in-laws though. I felt the same way. I think there is almost a loyalty issue or something there. I know I felt like they were suddenly all interested in our life and wanted to know things, yet, I felt they didn't really deserve that info/attention. They had never had the interest before and were not that close to us, however they felt somewhat entitled once I became pregnant. It used to freak me out. I don't know about your situation, but my in-laws, surprisingly, have had very few opinions about what my husband and I do in regards to parenting. They were a little more vocal in the beginning (first couple months or so) but have backed off in the last months. I think (or hope) that there is a certain level of respect there; we are raising a happy, healthy baby and perhaps even avoiding some of the pitfalls they fell into when they had children. I remember being SUPER sensitive to any comments that weren't glowing compliments of either my baby or my parenting for the first few months though.
What I used to try to do was plan out any interaction with them. I was not one to play "pass the baby" with my newborn and my husband agreed with me. We of course let the grandparents hold her, but for a limited amount of time and then she was either being fed or was put down to sleep. I would not allow any visitor to disrupt her schedule/needs. That used to give me more of feeling of control which in turn, made me less aggressive about protecting her. I also tried to not listen to any advice/comments made by someone who I didn't consider worthy of doling out mother-advice. I used to put on a pleasant blank face and try to not hear to the best of my ability. What I did hear, I just filed in my brain under "tidbit from unnecessary source".
That mother-protector instinct is a good thing; it is there to help you have the strength to protect your child under any conditions. I think it is just about keeping it in check and knowing when it may have run amok a smidge. It sounds like you are doing a wonderful job with your daughter and you should be very confident in your ability to keep her safe; you're on it! HTH