What is Gross Violation? And I believe I was set up to be fired- what can I do?

I had been with a big pharmacy for a little over 5 years and recently in February transferred to the store I just got fired from on December 19, 2010. Previous to this, I had been written up for taking my son to the ER and for myself being in the ER and being admitted to the hospital. Other than that, I had never missed any work. He said if I missed anymore work within 6 months, I could be fired. So I have been on eggshells hoping that no one in our household gets extremely sick again because I have been scared to have to miss work. I haven't missed any work since then. From time to time I had been "talked to" because he said he felt I talked too much.I came across some e-mails that my store manager sent to our district manager requesting pay increases and benefits to a part-time employee and said he had been taking his management training and that he was next in line for shift supervisor. I was promoted in February to Shift Supervisor and never recieved training. Just the past week before Christmas, they had the district manager and the loss prevention manager come to the store and interrogate me for over an hour making me feel like they were trying to make me admit to stealing, which I never did. To make a long story short, they put me on suspension on a Friday and said I could call Monday at 4pm to see if I could come back, but if the store manager found out before then he could call me and tell me. He called me Sunday and said they weren't gonna let me come back. He said he couldn't tell me the reason for termination and that the district manager told him not to talk to me about anything and to tell me to call HR. It's been a back and forth go around since. I cannot get a straight answer and no one wants to tell me anything. I think I was fired because he wants his friend in the position I was in. What should I do? Or what do I need to do? All I have found out is that they put my termination in as "Gross Violation" and he cannot even tell me what it means. What does it mean? Thanks.