i'm 17 and i have been sexually active for a few months now. i got my first depo shot on sept. 18 and had sex on sept. 30. is that enough time for the shot to become effective? lately, i've been feeling very tired, moody and i've gained some weight, like 3 or 4 pounds, but my stomach looks really bloated. i've also been extremely lightheaded at times and i get horrible headaches. they're so bad sometimes that i can't stand up and my vision gets blurry, then the headaches make me really nauseous. i experienced some light spotting on oct. 5, 6, and 7. but i haven't had a normal period other than that since i got on the shot. could i be pregnant?