What Causes A Delayed Menstrual Period With Negative Pregnancy Test?

I had protected sex with my boyfriend about 3 weeks ago, and I was supposed to get my period about the same time but I never did. I still havent gotten it yet and I was wondering if there was any chance I was pregnant. I took a test 2 weeks ago and Ot came back negative, so I don't think I am. Not to mention ever since I got my period I have been irregular and have missed months entirely (such as February). I am very active in sports and I am 18 years old. I just wanted some insight, thank you!