I began having nausea and mildly sore breasts about a week before my period was due

I had sex December 21st,2014 , I had my period the next day and continued to have sex through my cycle. My last time having sex was January 3rd 2015. I have a 27-29 day cycle (usually 28 days), I began having nausea and mildly sore breasts about a week before my period was due. I took a pregnancy test January 20,2015 and the results were negative. Today, January 21st my period(?) came on, however, I'm not experiencing any cramps like I usual do. The flow seems pretty normal although not as heavy as usual. Could I POSSIBLY be pregnant?