What Causes A Sensation Of Commencement Of Periods?

I used use the depo shot for birth control but I dont anymore and I stopped a March of 2009. Ever since I havent really had a period except for a time or two when my doctor gave a pill to make it come. Now out of no where I began feeling symptoms of an oncoming period. But all I've been doing is spotting and this has been going on for about a week and a half to two weeks. My question is why is this happening and could I be pregnant?