What Are The Side Effects Of Using Depo Provera?

I have been on depo provera since May 2009. No problems before - with occasional spotting with the first 2 or 3 injections, and none since. However, my last injection was mid March 2011, and I ve had spotting this May. First was on the 3rd May, with LOTS of BRIGHT RED blood, which slowed by the end of the day and became more like spotting and darker in colour (this was after a week of highly vigorous exercise), this continued for the following 3-4 days, then it stopped. I didn t exercise again until this week and again there is spotting, but it is a brown red color now (more like real spotting) - again occurring after exercise, which this time was only brisk walking in my lunch break. Is this normal?? What is going on?