This can't be ok... MIL taking over

Erin

This morning my MIL texts me saying she wants to take my 4 month old son for the day. I said maybe because I'm in the middle of sleep training and every time she has him it sets me back. An hour later my husband calls mad at me asking why I won't let his mom take out son. I told him shouldn't I have to even have a reason he's my kid. So he drives alllll the way home from work to argue with me some more saying I need to share him. I break down, cry and my MIL gets what she wants and takes him. Now I'm all alone, upset and feel like I have no control, no support. Am I missing something? I don't get why my husband and MIL constantly fight me. She always says I don't share and I'm hogging him. She gets him twice a week... It doesn't seem right.