Is it legal to take your child off of your health insurance?

I'm 19 years old, going to be 20 this month and my dad and I recently got into a little fight and he has now taken me off his insurance. A while ago he threatened it but I was told he was not allowed to because I have a serious medical problem. I was also told all his children are to stay on his insurance until either married or 26. None of my siblings or I have lived with my dad but he has provided us insurance since we were babies and it was never a problem then. All of a sudden him and his wife are making it a big deal and are saying I can't be on their insurance because I do not live with them. He has known about this for a while but he just decided to tell me today, the day that my insurance expires. He could have told me a couple months ago so I could get all my medicine and dental work done but he is being real immature about the situation. I also have a doctors appointment next week and now that I have no insurance I don't know what to do. Is it legal for him to take me off with no warning like that?