if your "GF" had a period at all she is most likely not pregnant, although it is possible to have a period while pregnant it would not be longer it would be shorter lighter most of the time just spotting. but if you talk to her about it and she feels it is a valid concern you can always take a test.
A period means she is NOT pregnant dumbass
you aren't old enough to have sex if you think a longer period = pregnant. take a sex ed class before jumping in bed again.
no, periods flush unfertilized eggs out. but since you're worried about it, tells us that you weren't paying much attention in health class when they wrapped the banana in a condom.