Wrongful discharge, discrimination or just unfair?

I was fired yesterday from my position of Team Lead and given the reason of poor performance. I know that FL is an at will state but I feel this is untrue and don't want to have this on my employment record. I have never been written up for any performance issues and I had 3 times the amount of work put on me as other employees. Why would you give 3 times more work to someone who has so called performance issues. When I express concern about why I had so much more work than everyone else I was told by my immediate supervisor that she pushes me more because she knows I can do more than the others. The only time my performance was discussed was when my immediate supervisor spoke to me in her office because it was brought to her attention by her supervisor that other employees think that she and I were not polite or even rude at times. This conversation took place 2 or more months ago and no more was said about it. My immediate supervisor makes more mistakes than I do and has a far worse attitude toward the other employees. I was let go by the VP of the location I work for and feel that if I am being let go for poor performance then why not her too?