Day on day off period?

I am waiting to go to the doc for a pregnancy test but wanted your opinions. I started my period a couple days late but I didn't think anything of it. I was on for a day (vey light) then the next day I had no bleeding. The third day i was light again and then it stopped. I feel sick, tiered, I have a headache. My have been a little sore but nothing too bad. If I'm not pregnant, does anyone have ideas about what else it could be? thanks!