I was in the same place as your GF earlier this year. My BF told me he loved me (after being together 3months) but i did not feel the same way at the time. We did have sex, though only once (which was my first time)
and after a while i decided not to sleep within again untill after i'd finished my disertation and exams as i was under alot of stress at the time.
He was in London at uni and i was at uni outsde of london which meant we only saw eachother a couple times a week, and i couldnt stay at his cos he shared a room with his brother and my parents wouldnt let him stay at their home so things were difficult.
Anyway, the few times i did have the opportunity to stay at his i either couldnt cos i was working early the next morning or i made excuses..why i wasnt sure at the time, but i think it was because i didnt necessarliy want to (issues with intimacy i think). After this he came out nd told me that he didnt feel 'wanted' by me and he also felt rejected. So we talked about it and i was honest about how i felt and afterwards we both felt better. He made it clear that it wasnt to do with sex and was happy to wait till i was ready.
However, nearly 2 months later i realised i didnt want to be with him and and kind of began to push him away and finally broke up.
Although i had issues within myself, he told one of his bestfriends, who was a mutual friend of mine and my bestfriends sister, that he was thinking of breaking up with me, which i heard from my best friend after her sister got drunk and accidently told her. Her sister was vague about his reasons, tho she thought it maybe to do with sex and the fact that after being together for 7weeks, i didnt want to stay at his and therefore he assumed i wasnt attracted to him. This messed with my head quite abit as i was falling for him at the time. Though, because i heard it from someone else i couldnt bring it up with him directly. so instead i kind of brought up the issue of whether we should break up at which point he acted really offended and shocked, which i took to mean he changed his mind etc. So after a few weeks i got over it, (so had thought) and after a while we slept together and things were great, tho suddenly i started to resent him and couldnt understand why, then a few months later we broke up, with him being completely baffled and thinking i was a nut case lol. I couldnt even give him an explanation because i was unsure in my self, untill i realised that i unconsciously never got over what he'd said.
If i could go back i would have told him that i knew, as he acted like he' was a martyr and i was 'just like every other girl' tho i was under pressure from my best friend not to tell him because it would ruin her sisters relationship with him. I still see him now and then and wish i could just tell him tho things are awkward enoughlol
So ur GF may either;
1 be not ready to sleep with u
2 not want to either because shes under stress or confused about how she feels about you...maybe you've done something that may have made her feel insecure? or from another relationship
3 you did say ur both going off to uni so maybe she's afraid that you may grow apart or something and so by not sleeping with u or saying i love u back shes trying to prevent herself and you getting closer/and or hurt.
Sorry 4 the long post, i gave way too much info but it actually felt good to write it down!! Good luck and i hope u sort things out x