Extremely Late period

I’m 17 almost 18, and I’ve been having pretty regular periods, around every 6-8 months it might change a little as far as come earlier or a week later. No big deal. But now I’m 15 DAYS LATE! I’m scared out of my mind and I don’t know what to do. I’ve taken 3 pregnancy tests and they all say negative. Within the past couple of days I’ve been having some symptoms, sore breasts, headaches, slight cramps. I don’t know what to do! Please help!!