In reading your question, a few things come to mind. A woman can have mid cycle spotting/bleeding and it can be due to a cyst which is common and not dangerous. Implantation bleeding happens but not as commonly as the internet would have you believe. And it's more like spotting and not bleeding. Chemical pregnancies happen and it is usually around when one would have gotten their period and is then confused for a period. Not sure if this is the case with your girlfriend. Early on like that, it basically feels like a period. With today's tests, women find out they were pregnant when years ago, women never knew. Anyway, your girlfriend wants to wait a day. Her choice, of course. She 'could' take an early response test which gives a result 4 days before you expect your period. I personally have a hard time being patient so am sure you really want to know. You could go buy the test yourself and give it to her. :>)
Symptoms are generally too subtle in early pregnancy to be associated with it. I would not assume her headache or such is related and actually, those same things happen with pms and stress.
So, go buy her a test and then she has no excuse to not take it. And . . . if negative, buy condoms. Always use them.
Implantation bleeding is typically more spotting compared to bleeding- however, it's possible given the time frame that you had unprotected sex around the suspected time of ovulation- that she could be pregnant. Many other things can cause bleeding at various times. If she is on a birth control pill, mid-cycle bleeding occurring isn't exactly uncommon.
At this point, get a test and have her take it- just to be cautious and get a better idea of what the result of that is. Other wise, I wouldn't be overly concerned about the mid-cycle bleeding unless she sees it start to become a pattern- then she can get to her doctor to have an exam.