No period and not pregnant...?

I went to my gyno back in sept or oct because I wasn't having a period. I did home pregnancy tests, the doc did one, she also did a physical exam and an ultrasound and blood work for my thyroid but everything was fine. she did see a little spot on the ultrasound but i wasn't pregnant. she gave me some pills and i had my period and i haven't had it since. I haven't had sex since before that first exam. any ideas as to why I'm not having a period?