What Are The Side Effects Of Depo Provera Injection?

I am trying to get pregnant. Since the depo provera shot Jan.1, 2010, my cycle finally started back Sept. 28, 2010. For every month going forward, my cycle has been starting 3-7 days early each month (ex. 10/25/10, 11/18/2010, 12/11/2010). I don't know when I am ovulating. Please help.