I am on Social Security, Can a job move from a position because i can't have full time hours?

I am working at a job, I got sick- went on leave, came back and essentially was forced out of the position I had for the last 2 years 6 months...I have SSI, Which only allows for up to so many Hours...they told me that because I can't open my availability to full time hours, that I was getting moved. I feel discriminated against because I am disabled,I have also a note from my medical doctor that states I can only work four hours a day... and feel that what they did to me was deliberate, because they know I am on SSI and also got the letter from me stating my knee condition... They then proceeded to ask me where, Cashier or Health advisor. I told them Cashier, when I went in to work to do the shift, I was told no your a health advisor. So my choice did not matter. IS there a way to find out what laws they broke...as I am really frustrated here, and wondering if this is a constructive Discharge issue.