Regrettably, there will not be an easy basis for a law suit if there is any at all.
The reason is that the leave that you were taking, while very understandable, was not a legally protected form of leave. Therefore, there are not federal protection for the leave. They also are not required to give a reason for termination.
The only potential legal basis for a claim against their behavior would be an oral contract
of some type or perhaps promissory estoppel.
For the oral contract, the problem is that unless you were promised something more than "at will" employment, the promise was really illusory. At will employment can be terminated at any time without reason, so if their promise did not include some statement that you could not lose your job, it wouldn't be a useful promise in which to base a contract
For the promissory estoppel, you could claim that you relied on their promise to your detriment, but this claim has little following in Texas (it is not statutory, but case law based) and, even if you won, the claim's value would be limited by what you employment status was...at will employment.
Finally, in either type of claim the expense is going to fall to you. This is not the type of claim that the EEOC or some other government agency would champion for you. You'll have to hire an attorney locally to do trial work. A trial by a cheap attorney can easily hit $20,000.00.
Edited by Jagcorps_esq on 4/4/2010 at 5:18 AM EST