My period is two months late!

I'm just becoming sexually active around August, after I had sex it was late but it still came. I had no period in October or November, I've taken nearly 10 pregnancy test and they all were negative there was no light lines  or anything it was a very strong negative. There are no physical changes, medical changes,anything. I've asked my guardian multiple times about this and she just said I was fine and to just relax. The last pregnancy test I took was December 3 and I'm pretty sure a hpt is pretty accurate by then. What should I do