No period without pregnancy??

I haven't had sex in 2 months (it was protected and he didn't finish) and had my period last month. Right now I'm 4 days late and I'm quite confused. I am currently under a lot of stress and have been for about 2 weeks. I'm wondering if I should be worried? Is it possible to miss a period without being pregnant? I should also mention I'm usually about 2-4 days late every month