When I was on the Pill, my periods were lighter. I’d never had problems with cramps, etc. to begin with, so I can’t tell you how it might have affected them. Everyone reacts differently, though, and there are so many kinds of pills, that it’s hard to say how it will work for you. It’s largely trial and error.
My last period was over a week early, too. From what I understand, though, being sick and stressed (both of which I was) can wreak havoc on one’s cycle even if you’re not doing anything else, like using birth control and/or having sex, that can account for a disruption. Since it’s only the second time it has ever happened- in an eleven-year plus time span- I’m not too worried about it yet; it doesn’t seem that it’s going to show up early this month.
Yet another recommendation for your girlfriend to see her doctor. It may be nothing, but it may be something, and the sooner she finds out the sooner she can do somthing about it.
Anecdote time – my mom was having her periods every two weeks, and ended up severly anemic. Her doctor put her on birth control pills to regulate her menstrual cycle, which helped out a lot.