Confused

I had my period for 3 days in July and it wasn't heavy pretty light . I've never had that happen , my period is usually last for 5-6 days , took a test 12 days later and it said Negative. Didn't have my period at all in Aug & now we're in September with no signs of my period . I've had unprotected sex with my boyfriend but I don't have nooo pregnancy symptoms and he only came in me once other times he pulled out . Could this just be stress or something else ?