Are Light Spotting And Period Symptoms Without Periods Signs Of Pregnancy?

hi, i've taken two shots of depo...first in August of 2010 then in Nov 2010. i was due to take the next shot in feb but decided not to want to get pregnant. I got a spotting on the 12th of feb, then the week leading up to the 12th of march i had period symptons but no period. I've been having unprotected sex, i've taken several pregnancy tests but all neg. Could i Be pregnant still?