What can I do about my job?

So I have a medical condition where sometimes if I stand for a long time I begin to feel very dizzy & light headed & on some occasions nauseated. Well Sunday was one of those days...I hadn't eaten anything since 10 a.m. that morning & I had a pounding headache. This WAS information that I disclosed in my interview so they are fully aware of it & there are supervisors there that will ask me how I'm feeling. Sunday I informed one of my store managers that towards the end of my shift (I was to clock out at 8) that I wasn't feeling well & I needed to go home because I was fine all day but around 6 that's when I started to feel sick. She goes "Okay we'll see what we can do" I got ready to clock out & one of my CSMS go "No you have to stay because we're shorthanded & I have no one to replace you" & made me stay on the clock. I felt like I was getting worse but he wouldn't let me clock out. I ended up clocking out 10 mins earlier than I was supposed to but my supervisor told me I shouldn't have clocked out at all because I was told not to, but I was sick! I was not gonna put myself in a position to be hospitalized just because they were shorthanded & couldn't replace me for what? 30 more minutes? & we weren't even THAT busy. So my question is, what should I do in situations like this? Are jobs even allowed to do that? I mean, what are my rights in this particular situation?