Is this the Depo Provera Birth Control Shot? They should give you a pregnancy test any time you start on a shot or BCP's. I know when I was on Depo my Dr did a pregnancy test before every shot. If it is Depo, then it has crazy side effects, it does weird things to your body. When I was on it I bleed for almost 3 months straight. Try googling the name of the birth control shot you got, it will give you a list of side effects and you can see if what you are experienceing is a side effect. Also if it continues you may want to contact your Dr.
i got my first depo shot and kept bleeding as well so i never went back and got the second shot 3 months later i bled for 8 months straight went to another doctor and she put me on pills and it stopped the bleeding, my cousin is on the shot and has had no problems with it...my best friend on the other hand was on the shot and got pregnant...