Morning sickness and work

I’m a teacher at a private school and I just told my headteacher that I’m experiencing morning sickness and that I might not be able to finish the school year. I told her to look for a teacher and that I’m staying until they find one. She got very angry and made me feel that I’ve done something wrong. She mentioned that she asked me at the beginning of the year about pregnancy and that I said it was something very unlikely because I’ve been trying to conceive for 5 years. I didn’t expect to get pregnant and now I can’t even feel happy about it. She didn’t even tell me what was going to happen and if they will look for another teacher and when. They don’t have anything in the contract about leaving without notice and when they don’t like a teacher they’re not ashamed of firing them right away. Have I done something wrong? I’m trying not to cry :(