Can you get fired while on medical leave?

The first question is whether you took leave under the Family and Medical Leave Act (FMLA) or its equivalent state law (many, but not all, states, have a similar law). FMLA and similar laws generally require that the company be large enough to be covered--e.g. at least 50 employees within a 75 mile radius--and also that the worker seeking to use the leave have worked there on a more-or-less full time basis for a year. If either the company or you do not qualify under FMLA or a similar state law, then there is no legal protection for taking medical leave: that is, as a general matter, unless required by FMLA or a similar law, employers do not need to provide medical leave and may fire an employee who is out/misses work for medical reasons.

Even if FMLA or a similar state law is involved, all they do is protect you from being fired for taking the medical leave--you could still otentially be fired for valid unrelated reasons (e.g. violating company policy, poor performance, etc.) However, from what you write, if you took leave that was protected under FMLA or a state law, you would seem to be able to make a good case that the firing was illegal retaliation for using protected leave. In that case, you would have a legal claim or cause of action.

The second question is whether you have any sort of employment agreement (including a union agreement) which gives you the right to medical leave; if so, that contract is enforceable.

So if you either took FMLA leave or leave under a similar law in your state, or had the right to leave under a contract, then you likely would have a cause of action for wrongful termination. However, other than that, you might not have any claim, though you still should consult with an employment law attorney (many provide a free initial consultation to evaluate a case) about your situation in detail to be sure.