Is your girlfriend on birth control? If so, then her periods might just be acting up a little bit...Birth control can play some mean tricks on a woman's body sometimes. :3
but if she is not using birth control, then do you know if her periods are irregular? Assuming she is 16 or 17, about your age, then like some teenage girls, her periods may be a little haywire. But i'd definitely say that the brown discharge can be her period. It's pretty close to being a 28-day cycle (28 days would be july 22nd). The discharge being brown can just be a sign of old blood being expelled, which is nothing to fret over. Don't worry, the same thing happened to me; I had a similar [heavy] discharge for 3 days and then my regular period followed, and I was okay...Her body might just be acting up, but it's nothing to lose sleep over. (forgive me for being graphic and using my own experience as a reference xd) one more question...Is your girlfriend stressing out a lot about the possibility of being pregnant? Stress can also alter a woman's period in a number of ways.
Here's my advice (i'm going to steal some of your thunder now, x0x011...I hope you can forgive me 8) )...Tell your girlfriend to not freak out over anything, and wait until next month to se if her period goes back to normal. If she misses it, then have her take a home pregnancy test. Or, if you do not want to wait, see if you can have blood testing done; these tests can be done confidentially, so you don't need to tell your parents or her parents about it. These tests are foolproof and will give you the definite answer.
One more little piece of advice...See if your girlfriend would be interested in using birth control. In the event that the condom breaks or falls off (again >.<), you will have a back-up method to hopefully rely on. See if there are any planned parenthood clinics in your area; they will give free condoms and birth control to anyone under 18, and they'll explain everything you need to know. You may also benefit from going and having them explain things to both of you at once (just a thought...Take with 1/2 grain of salt, dear ^-^).
I know that was really long-winded...Sorry about that. I wish both of you luck in your time of worry, and I pray you guys get the answers you desire.