I think that’s your basic instinct kicking in because someone has just took your baby and blocked you out as I would be the same and be really upset, have a chat with your partner to ask if he can talk with his parents about appropriate boundaries as that’s not on at all
I agree with @Becky (Can't see waves), that was rude, a normal person would have asked to take your baby hopefully with the rationale of giving you some time to yourself to rest or do self care, but the snatch grab approach nope. Are you living with them?? Some boundaries needs to be put in place, asking permission and stating intent/ plans while baby in their care. Your hormones will be all over the place but not everything can be blamed on that, your a mum and your always going to be in protective mode. Some people think because they done it before means they know better. I’m hoping after 3 years your partner is someone who will listen and speak to his parents and be on your side.
I'd say it's hormones. Your hormones have not yet settled at all and will still take ages. I think up to 2 years. Maybe do discuss with them that it makes you feel anxious and youd appreciate if they ask or warn or arrange with you first before just taking him. Tell them you don't mind them taking and totally see how they want to help you by giving you a bit of a break and you appreciate it, but not just to take an go. Ask. And in a way prepare you to say goodbye and know he will be with them for an hour or 2 and that will ease your anxiety more.
First of all that is very rude of your FIL to just take your baby out of your arms. Why are they taking your baby into another room and shutting the door? I would be really cross at this. As I would want them in the same room so I can still see my baby. Also in the first 2 years your hormones are all over the place. I used to get upset over things I wouldn't have before. It's normal x