Period MIA

Ke
When I fist started my period I was really irregular(about the age of 12-18), but in the past year it has been more and more predictable( I just turned 19). However, I am two weeks late for my period. I am sexually active, but we use a condom almost every time. I took a test but it came back negative. I don't think it is related to stress. Does anyone have any ideas what could be wrong?