Could i be pregnant?

Ok...so i had a baby 3 months ago and my doc put me on the birth control shot.since then i have had unprotected with my boyfriend (who is the father of the baby).every time we have its unprotected.i did get my period...but before i did i kept getting stomach pains,backaches,had to pee wayy more...and was felling nause.i have taked HPT but all came back negative.but i keep having stomach pains and etc. Someone please help? Could i be pregnant even though im on birth control?