I'm pregnant and my boss is knowingly causing me stress. Is there legal action I can take?

I'm almost 5 months pregnant now, but for the last few months my boss has literally been "picking" on me; doing little things here and there that he does to no one else but me. My main concern is that after all of this and sticking it out for months hoping that he'd move on to someone else, I heard from a fellow employee that my boss ranted about me and said that he was purposely going to try to make me quit (thus he doesn't have to worry about unemployment). I gave my boss a doctor note when I first found out about the pregnancy stating I needed frequent bathroom breaks, water breaks, 15 min breaks every 4 hours, and that I can work no longer than 8 hours a day. He works me 9 hours and has a tone/look of displeasure when he finds out I take an hour break in full or even intervals (when work is slow). I've worked at this job for 3 and a half years. I'm a store manager for 1 of 2 stores. Working there was great, and then I got pregnant. (I live in MI, work in OH)