Light period then heavy 7 day period

my girlsfriend and i had protected sex on the 16 her period was due the 21st. this was our fist time and her period didnt come til the 22nd i red that the first time you do it your period could be changed a bit so i guess it ruled that out. when it came it was pretty light and not alot. the next day it was a regular period normal heavyness and amout but it lasted 3 days extra then how it regularly was, the 3 extra days however were pretty heavy too she went through about 5 tampons a day. it seemed to go away but then the 8th day it came back in a small amout there WAS some brown in there but i belive it wasnt much at all (wich im scared of because it connect to implantation bleeding, though she read about it and said it was nothing like it.) im pretty scared if shes pregnant it will be a huge problem. but i just want to know your opinions if you think she is or not. is about 6 days of heavy bleeding enough to say shes not pregnant? how long is too long for a period? thanks you very much