I don't think you can sue for being absent. Employers need employees to be there. You have a right to the FMLA time, which you used. Doesn't matter if they believe you are sick or not, you are not going to work and that is a valid reason to fire someone. Sorry.
Even FMLA is for larger companies, not for small companies. As far as disabilities they have to offer you a safe environment, like changing your chair etc but they don't have to keep you if you take too much time off. Otherwise, everyone would do it, right?