I had sex (protected) with my boyfriend Sept. 8, and started my last period on Sept. 9. We left for college shortly after that, so haven't been together since. It's now Oct. 13, and I don't have any signs of a period. I've been irregular in the past, but not when I was sexually active. Since I haven't had sex this cycle, should I be worried? I've been very stressed with school starting again.