Unfortunately, anything is possible. Could be v v unlikely but the only way to clear it up is to do a pregancy test as pippa suggested. That'll stop you both worrying.
Any chemist will sell them - I would suggest getting 2, rather than just 1, and testing both at the same time to avoid the minimal possibility of it being an incorrect result. I scared myself stupid with a dud test once upon a time.
If funds are an issue and you can't afford to buy these kits or are too embarrassed to, she should go to the GP who will do it for free...although oviously, this will take longer to book appointment etc etc.
As she's over 16, the GP also HAS to keep it confidential so parents won't find out. I think family planning clinics do the same thing too.