I might possibly be pregnant but I'm still having a period. Looking for help on what I should do. Suggestions?

I am fixing to turn 18, I was stupid and had sex one time. It has almost been 3 months and I'm still having a period, its not heavy but not light. I have been having cramping and a lot of tightness in my lower tummy. A LOT of tightness when I do heavy lifting or bend over, kinda like my skin shrinks and stretches when I lean back up. My lower tummy also feels a little tougher...almost harder. I'm not sure what to do because a test comes back negative. I was thinking that if I'm having a period, the hormone that a pregnancy test uses to determine wont show up. I'm 17 at the moment and even though its my fault, I would like to keep this from my mom and just wonder what I should do. Anything would be helpful. Thanks.