Late period & negative home pregnancy test.?

Last period was April 16, it's now May 19. Took a test on May 17 and it came out negative. I'm not stressed, I'm not over- or under-weight, I'm not on birth control, I haven't started any new medications, and I haven't been sick. I'm 19 so it's not pre-menopause. haha. They are the only other reasons for a late period I can find besides pregnancy. I figured my HcG levels would be high enough to detect when I took the HPT, it was 2 1/2 weeks after the last time I had unprotected . Now I'm just confused. Where do I go from here? Should i wait and take another HPT, or should I just set up a doctors appointment? Any suggestions/help?