Suggest Remedies For Prolonged Absent Periods

I had unprotected sex about a month ago, and went to a clinic this past week to get a pregnancy test. It came out negative, but I still fear that I might be pregnant. Although it goes against what I know of science. I am just wondering if something else could be wrong, because I hadnt had my period since November of 2010 but had sex in January this year. I was wondering what I should do and if it could be thyroids or something else.