Pregnant???

I'm 19 & my period is 25 days late. My periods are not normal & never have been. I've never been sexually active at all up until late December 2016. I am sexually active with no birth control (Trying to keep it a secret from my mom, because she would kill me. I have a doctors appointment to get on birth control next week!!) I took a pregnancy test two weeks ago, & it said negative & then about four days ago, I took another one & it said negative as well. But I'm still confused as to why I don't have my period? I was under the impression that sex helped regulate your period.