I received my fiirst injection at the end of jan, my second injection was late by about 2 weeks. I have been spotting mostly dark brown for about 3 weeks now. Its never heavy enough to soak a pantyliner but it is very aggravating. Sometimes I feel like I may be pregnant but I don't know if that's possible. Can anyone tell me something?

No because is 100% effective. And if you were late getting a shot, and had unprotected sex during that time, your chances of pregnancy are even greater. Play it safe, take a pregnancy test.

Play it even safer--call your doctor.