What Causes Inconsistencies In Menstruation?

I am 19 years old.My periods are regular, in Oct I got a very light pinkish brown discharge. I took a home pregnancy test and it was negative. So I waited. I was supposed to get my next cycle Nov 6 and it didn't come, I waited another week took at test it was still negative. I am now around 22 days late. I had an ultra sound Nov 7 to see if I had any ovarian cysts due to some pains and that came back with everything normal. Any idea whats going on?