You seem to be very certain you're not going to dump your gf, so being with the other girl isn't really an option.
I think maybe you're drawn to her coz she's a "new" friend, and you've spent quite a bit of time with her and there's been a lot to talk about and to know about her etc. Its how friendships are at the beginning.
Like you said, you're going to have great times with your gf after exams, so I'm guessing you won't be spending so much time with the other girl anyway.
Maybe you shouldn't really meet up with the other girl so much. You could be sending out the wrong signal by being so friendly with her and always seeing her, which may backfire later.
Your gf is like any other girl out there and is bound to raise an eyebrow if you keep meeting some other girl. What probably makes it worse is, she's a new friend of yours, and not a mutual friend who your gf knows or can trust. She may think that she's not had enough time for you so you've found that company in someone else. I don't think constantly asking about the other girl is going to do any favours to your relationship, so maybe that needs to come to a halt too.