I went on depo last year. My gynocologist didn't inform me of any of the possible side effects of the drug and i, unfortunately, did not do any research myself. I didn't have any of the cramps you are describing, but I did spot for almost the whole time I was on it. By the second month, I knew that when the time came around to get my next shot, I was going to go back to being on the pill. I have a number of friends that are on depo and love it, but my experience with it (the spotting and the fact that I was much more emotional while on it) was a very unpleasant one. You may have already done this, but, if not, there are a number of web sites dedicated to women talking about their experiences with depo--you should check them out (just go on google and do a search). There are a lot of negative side effects that depo can cause and it's good to learn about them.
Good luck.

Yeah like bleeding real heavy on depo for like up to 6 months