Late period

jessi

I am 14 days late. My period has usually always been irregular, but I’ve never been this late or missed a period. I’ve taken 2 pregnancy tests that have turned out to be negative. I’m 18 years old and my mom keeps claiming that I’m pregnant. I don’t want to tell her I’m sexually active which is why I haven’t told her about the pregnancy tests. Overall, I just want to find a reason why I’m so late. I’m not on hormonal birth control either.