Late Period but Negative Tests - HELP!

About a month and a half ago, I had unprotected sex right around when I was ovulating. About eighteen hours later I took plan B. Two days later, I got this pseudo period, which was very light and only lasted three days. (Normally, mine are pretty heavy and last at least five days.) I figured it was my regular period, just a little off because of the pill. Now, over a month later, I'm over a week late. I took two pregnancy tests, both of which came back negative. What's worrying me is that I haven't gotten my period nor am I having any PMS symptoms. My boobs are sore and I'm mildly nauseous at random points throughout the day. Is there a chance I could be pregnant even though I got this false period, took the pill, and the tests came back negative? What should I do?