am I pregnant?

Around a month ago, my boyfriend and I had unprotected sex and a few days later, my period came a bit early and it was extremely light. I took a test a few weeks later and it came back negative, though I don't think I let it sit as long as it should've. After the negative test, I've still been having a few symptoms (mainly hot flashes that come and go for a majority of the day) and nausea. I've had a few headaches. My period is late by a day, which is a little abnormal for me. Typically my periods come right on time. Could I actually be pregnant or is it just stress causing my period to be late?