alright so i need some serious help!

i am 20 years old and am not on bc but my periods have never been regular 

so in august i had a scare so i took plan b on the 12th this caused my to not have my period in september so i was freaking out! but i thankfully got it in october on the 12th. i was not worried until my boyfriend came to visit me recently, he came the 9th and i had not seen him since august so yes we had sex. i told him to check and see if the condoms were ok and they were but i did not get my period on the 12, i was not worried to much because it is never regular but yesterday i had sort of brown and light blood but for only like an hour and now not so much! i took a test and it was negative. but im still freaking out! 

i know brown blood is old blood and i do eat was differently at home than i do at school, and im at home for break right now! and i also work out a lot and lost a pound so idk if that has something to do with it! or the fact i had not had sex in 4 months maybe i was to dry or he cut me and its just now bleeding ha idk

please help im freaking out!