I'm on birth control and the past couple of months, my period has been coming on with the first green pill. This past month, it was a couple of days late and it only lasted for about two days. It was not as heavy as normal. Now, i'm having pains in my lower back that have been there for about two weeks now. I usually get back pains a week before my period. I've also been having slight headaches and a little nausea in the middle of the night. I take my pill daily, but not the same time everyday. Could I be pregnant?