She's most likely not pregnant but I recommend she take a pregnancy test just to be sure.
I also recommend you get some condoms or she get on birth control so you won't have to worry like this about an unplanned pregnancy.
It's not that uncommon for teen girls to have periods like this. Some can be right on time every month, some can skip periods for months and some can have more than one in a month.
The tests can be bought at most any store. They have them at Walmart for under $1 so you need to get one and while you're thee you should also get some condoms.
I hope everything works out alright for both of you. Let me know if you get the chance.