Your Advice Is Needed.

Well, I am 16 years old and I have been sexually active for almost a year now. I have been with my boyfriend for over 9 months and I think I am pregnant. I haven`t had my period for almost 2months, but I took two at-home pregnancy tests and they BOTH said I was not pregnant. If I am not pregnant, then why is my period so late?? (NOTE: my period has been about a month late before in the past, but I was not pregnant.)