Period one week after last

So this has never happened to me before. I had my period last week (4/11/16 - 4/14/16) and now I have it again (4/21/16) or at least think its my period. I have never had this happen to me before and I'm a little freaked out. I'm 21 and I am not on birth control. I have never been on birth control. My boyfriend and I are usually very careful, although there were a few times we had sex without protection. I took a pregnancy test and it came back negative. Anyone have any ideas or any advice? I'm going to try to make an apointment with a gynecologist tomorrow to get checked out but I'm still freaking out.