Spoke up about favoritism, went on pregnancy leave, on day of return got laid off.

I want to know if I should look into this matter furthur on how I was unfairly treated. I had been at my job for 2 1/2 years and was promoted and received 3 raises so I know I was a good employee. My problem started when I needed to start my prenatal care doctor visits. Also, it just so happened I had to take my daughter to her doctor the same week. I was told I needed to fill out a PTO (Paid Time off) form when I was not there the whole day. I was a salary/exempt employee and did not agree that I should so I questioned this. I asked my supervisor and he said there were "no exemptions- when you don't work a 40 hour work week, you fill one out". Other employees were never asked to fill out this form unless they took the whole day off, so I went to our Human Resource Dept.. I mentioned how EVERYONE is coming and going, leaving early, etc. and they never fill out a form. I was told that yes, he was wrong. After this, my supervisor did not speak to me. He ignored me and didn't even let me do my work. He asked others to do it which bewildered me. He knew I was getting ready to go on maternity leave. When my weekly visits to the doctor came, I said that if I was asked to fill out a PTO form, that we needed to talk seriously. I need to mention that there were 2 other pregnant ladies there at work at the same time, and I made it a point to ask if they would be asked to fill out a PTO form. I got no answer. I guess I did make it sound like a threat, because I included EOE on the bottom of an email I sent. This did not sit well with our HR dept. Next thing I knew, I was called in to a meeting. This meeting was just me, our HR head, and a "medium". I asked if I should have someone present as well, and they made a comment like it was nothing at all and no need and laughed it off. I was literally interrogated as to why I was so unhappy, why I thought I was being treated unfairly, etc. etc.. I explained to them how I felt I was singled out and after about an hour, I was stressed and upset. (I was 8 months pregnant, and my hormones were at their peak.) I was even asked if I was so unhappy why don't I look for another job. I said I was in no shape to look for one (she denied this comment the next day, by the way), but that I would think about it later. Eventually, I got up and walked out. The HR head said I was not dismissed. I simply told her I was in no shape to talk -I was crying uncontrollably, and I left. The next day, She called me for another meeting, and presented me with an 'insubordination letter', for walking out. I told her I would not sign in because it was not what had happened at all. It did not mention the questions that made me upset or anything to that effect. To top it off, she told me that I could go ahead and be off to go with my daughter to the doctor, but I would have to take the whole day! I was forced to use my PTO hours. Is that discrimination or what? My co-workers were as baffled as I was. This all happened about 3 weeks before I was to go on Maternity Leave. At this point I was just tired of the whole mess. I told HR that I would willingly fill out a PTO form and take off the whole day for the remainder of my weekly doctor visits because I was tired of all the discussions and meetings. NOW she said, it was okay with her if I came in late, that I didn't have to fill out a form. I had informed her too that I wanted to take an extra 6 weeks off with the Family Leave Act. So we got all the paperwork done and that was it. Now here's the icing on the cake. The week before I was to return to work, I received an invitation to our company Christmas party which was on a Sunday which I did attend. Everyone mentioned that they were happy I was coming back. My day of return is the next day, Monday. I walked in and I'm told HR wants to see me. I went to her office and she asked if I had fun at the Christmas party and so on. I thought we were going to go over my key-less entry code, etc., etc. Next thing she says is she doesn't have good news for me. That they were terminating my position. I didn't really react much. I said okay that I kind of expected something like that. She said she tried to call me the week before. I seriously doubt it because I have caller ID. So all I got was a 2 weeks paycheck and my insurance paid till the 31st of Dec. (total 3 weeks). I proceded to go back to my desk and my co-workers told me how delighted they were to have me back, that everything was the same again. I told them they're letting me go, and their jaws dropped. I wish I had a camera.
Now I get a letter in the mail asking me to sign an updated form to "Comply with Texas Law" for notice of discrimination and workers comp. Exibit #3A. If I'm not employed anymore, why are they asking me to sign this form. Do you think they're trying to cover themselves just in case? I really need some advice. Or should I just let this go. It's true I wasn't happy there, but I was a very good employee. I loved to work and help out anyway I could. I know something happened and I guess I really got under their skins. My supervisor and I use to have a good relationship too. The HR lady is always phoney so I never knew with her.
Please let me know what to do. Any comments are appreciated.
Thanks!