Ok, well, first of all, for the first 1-3 months of starting birth control you still need to use another form of protection, like condoms. Her body needs to get used to the hormones and adjust to whats going on... Information your girlfriend should have been given by her doctor. Because of what time of the month it is and how soon it is after your girlfriend finished her period that you guys had sex, I wouldnt be extremely worried about her being pregnant, but it is a definite possibility.
Basically, so you don't have to be worried every time you have sex, for the first 1-3 months use something else like condoms... Oh, and try to make sure she takes her pill at the same time everyday.