What Causes Brown Vaginal Spotting Along With Abdominal Pain While On Depo Shot?

Ive been on the depo shot since this past august. The first 3 months i got no period at all, no cramping and no spotting. I got my second shot in november and had unprotected sex. I now have bad brown spotting off and on for weeks and cramping, abdominal pain every now and then, and lower back pain not as often. Could i be pregnant? Or are these just side effects of the shot?