No period for 7 months and not pregnant?

I had my second child feb 15 2008 and in march of that month I got my very 1st depo shot. I had never been on it before so I thought I'd try it. I'd still get my peroids like clockwork (they've always been very consistant and on time) but after 3 months my periods never came back and I also didn't get the next depo shot. I'm going to the doctor tomorrow but I'm afraid its something serious since I've had cysts on my ovaries before and have had abnormal paps. I have cramps every day, I've lost the ability to gain weight, and no drive. I'm 25 and 105lbs, I also don't smoke. Has anyone had the same problem??