Why? 😩😩😩

So I recently got hired into a place. They are fully staffed but manager was willing to still hire me in. 2 people there are still working there but just haven’t been there due to covid. Now I’m dealing with medical issues and the problem was that I had to call in once for vomiting and being severely nauseous. I’ve been dealing with being sick on and off for the past few months not knowing what’s really causing me to be sick. I’m supposed to work tomorrow from 7am-3pm but the only time my doctor was able to get me in was tomorrow at 1:30pm I told my manager that I can work a decent portion of my shift but I’ll have to leave early for a doctors appointment (even said I could get them a doctors note for whatever is going on) and they aren’t too happy about it. She didn’t really give me much of a response and I’m worried she won’t let me leave to be on time for my doctors appointment. Can she refuse me to leave for my doctors appointment when I come in and work a period of my shift? What should I do if she does? I need this job for rent and shit but I also can’t control being sick and it going on for as long as it had been going on for.