MIL problems

So I used to let my MIL watch my son until I walked in on her and saw her basically try to force feed my son his bottle. He was screaming and crying because he didn't want it and she kept just putting it in his mouth. Well now I will not let her watch him and I've noticed recently he hardly drinks his bottle anymore and I'm thinking it's because of that. I haven't told her or my husband why I won't let her watch him so I just say my mom is going to so it doesn't cause conflict because we live right next to her. I'm just wondering how to get him to like his bottle again and if anyone else went through this?