Standing and running at work

So, I work at Walmart, and my job has me running all over a full super center- including the back room! So I’m on my feet, walking or running for 8 hours a day. I’ve been having really bad cramps when I go too fast, so I tried to tell my manager that I need to be able to go slower. She said she can’t move me in our department or to another one until I get a dr. note. Like, I get not having me move departments, but she won’t even let me do a lighter job within the department. So I’ve been crying all day because 1.the pain of the cramps plus 2. Hormones and 3.being told I have to suck it up, and they might move me if I get a dr note. Has anyone else had employers like this?