I always and constantly marvel at the degree of passiveness exhibited by employees, and the total surrender of control they give to an authority figure. It kind of reminds me of the **** thing, "I was just obeying orders!" At some point, we have to be responsible for our own selves and our actions.
If you were really having the worst headache of your life and your symptoms were so awful that you were convinced you had something terribly wrong with you, then why would you abandon control of your life, health and destiny to this one stupid supervisor who was not concerned about you, was totally self absorbed and involved with getting the work done, and had no way of knowing whether you were having a true health crisis except for the feedback you were giving to them?
When you decided to hang on a little bit longer, they probably thought they were practicing good management, calling your bluff. Okay, so they weren't compassionate, weren't aware of the implications of your symptoms, weren't proactive about your health issues. But after a certain point, I think it is the responsibility of the employee to say, "Sorry, man, I'm too blasted sick, and I'm leaving!" and let them do their worst. They did not refuse to call help for you, fail to get treatment for you.
I once saw an employment situation where this happened, a worker literally fell over on the floor having a stroke and the employer and other workers ignored him lying there for a couple of hours. That employer got sued.
To want to sue them because they weren't nice to you when you told them you were sick doesn't seem reasonable or based on principal to me. As I said, you weren't fired, you were just threatened, and you can't prove that it was their fault you didn't get treatment sooner. That's the real principal of the thing.
If you didn't qualify for FMLA, they could've fired you the minute you left the building and it wouldn't have been illegal. Mean, maybe, but not illegal. But they didn't. Your being eligible for FMLA, you probably began it at once when you left and sought treatment, and then they couldn't fire you, at least for the 12 weeks.