well im 20 and I just started taking Ortho Tri Cyclen Lo for the ...

well im 20 and I just started taking Ortho Tri Cyclen Lo for the first time..I never took birth control before,I started the pill when i was suppose to like my doctor told me to and i take the pill everyday at the same time but yet every time i have sex hrs later i start bleeding.Its not really heavy but its there.It also would start then stop then come on but be really light.I took a pregnancy test just to be sure even though i didn't think i was pregnant,SO WHATS THE PROBLEM????IS IT THE BIRTH CONTROL OR SHOULD I BE WORRIED?????????????