period ended early

i had a three day long period this past week (started saturday 8/27/11, ended monday 8/29/11) it is getting me scared because it usually lasts five days. i did have a period that started on 7/21/11 and ended 7/24/11 (a day after i got really drunk to a point i was throwing up, this too was three days long), this happened 10 days after when i thought was my regular cycle ended. i have on occasion been having unprotected sex with my bf and one night (early august, cannot remember date) we had a close call and he said that he may have released a little bit inside of me. the following morning i got plan b and took it as instructed. i was not getting a period on 8/21 or 8/22 (when i had expected it based on the second period that occurred in july) so i took a pregnancy checker and i tested negative. i got a high fever and a sore throat on 8/24/11 which subsided on 8/26/11. my bf and i had unprotected sex that night (8/26/11) but he did not release inside me at all. saturday evening (8/27/11) my period started. that night we decided to have protected sex (after having read from several sources that it shortens the duration of the period). But a period that lasted only 3 days was way too short of a duration. i tried a pregnancy checker tonight (8/29/11) and i tested negative. but i am really freaking out, can anyone help me out?