If I recall correctly, the OP is a bagger at a grocery store. There's been a lot of good advice here but I don't think all of it is applicable to her job position. Going home sick (saying "that's not possible") is in an office setting in one thing but is totally different in this type of job. Saying this or something similar could cause a cashier or bagger to be let go. Scheduling adequate coverage for cashiers and baggers is already difficult. It becomes more difficult if the OP becomes too ill while on the job or before her shift as there is no longer adequate coverage. Now the customers complain and possibly, Corporate comes down on Management. Ultimately, Management probably views cashiers and baggers as easily replaceable.
So I can understand OP's frustration with that part of management not fully understanding or not wanting to deal with the issue. With OP's limitations, there are not many employment options. It is also possible that Management is frustrated with the OP - not necessary herself personally but with the situation, i.e. (thinking like a Manager for a minute - she (OP) has health issues and illness can be a serious problem but illness can't be predicted. I'm trying to work with this with the scheduling but it is not always easy and I can't always give her what she wants or needs becuase I also have to accommodate person X and Y too.)
I do think the OP should avoid any negative attitude at work; put all requests in writing; and get into contact to HR and Corporate. I don't think the OP will change anything with Management at this point.
I really agree with you. I think the OP, at this point, has nothing to lose by going to management and HR. Even if she doesn't qualify for FMLA, there could be the possibility that upper management or HR are willing to work with her medical issues. And if she gets the support of the "big boss" and HR, her supervisors can be as snide as they want with her, but she has their superiors as back-up.
When she needs to leave, she can tell her supervisors, "I am getting a migraine and I need to see my doctor immediately" and they can snark all they want, but she can go. Later, she can email HR or the big boss (if she has already obtained their support) and say "Just to let you know, I had to leave due to a migraine today. When I informed Supervisor they said..."
So the supervisors, even while they may never be pleasant, can't fire her or stop her from attending to her medical needs because they can't override the decisions of the higher ups.
Maybe OP won't be able to get any further assistance from management or HR, but she's at least got to try.