Why is my period so late

The last time I had my period was the start of September, it is now November and soon December. I took a pregnancy test on Friday and I’m not pregnant, I took a different pregnancy test to double check yesterday and I’m STILL not pregnant.

The last time I had sex was about 3 or 4 weeks ago.

I’m really worried and I don’t know why it’s so late. Someone help? Thank you.