We've had so many threads with the same fear, I swear it could be its own forum....
First off, while the
normal times for ovulation is about 2 weeks before an anticipated period, that is not cut in stone. It can occur almost any time in any given month, up to one week before the start of a period. This is called the 'luteal phase'. Because your girlfriend is young, her cycle is probably not totally regulated, which brings me to this....(an oft-repeated mother-mode of mine)...
Why, for all that's sacred, are you having sex if you are going to have the least amount of panic over the remote possibility of a pregnancy? If you are not ready to be a parent, you need more than just a condom.
Are you aware that ovulation is not just a one day occurence? From start to finish, it can cover 4 days. Are you aware that sperm can survive for up to three days, just waiting for that little egg to appear? *gets off mother mode.
Get a pregnancy test. There are early symptoms, but none appear before any pregnancy test would confirm or deny.
Get your girlfriend on birth control. Then get educated. Panic comes from not knowing how to prevent a situation that would cause that panic. And if she's not old enough to get birth control, get a lawyer.