This is hardly a surprise, if I’m honest. People at university (if I’m any example, anyway!) are still experimenting, and are often having their first serious relationships. It’s often the case that one party might want to take things a little more seriously than the other; for one person it’s “love of my life” material, for the other it might be “sure I like her, but I don’t see it lasting”. Doesn’t matter on the sexuality, it’s just a basic human condition.
As you get a little older you learn that the “drunken fumble” that we’re probably all guilty of at some time or other just isn’t worth it. And if you’re the person being cheated on you have to decide whether it’s a one-off (which you might decide that you can forgive) or symptomatic of a deeper issue. Maybe she’s decided that you’re just not the right person, or that she’s not really ready for a committed relationship.
Either way, you’ll not be the first person to get their heart broken at university. I just hope it works out OK for you, whichever way it goes.