Dude, I got mono from a drinking fountain at a flea market in Florida, so you need to relax and quit being so insecure about your girlfriend. She didn't cheat on you, she probably just walked into a sneeze cloud by someone who has the disease.
You'd better watch out though, because you can still get it from her long, long after she has recovered. I had my first case of mono in 1993 and then got sick with it again in 1998. Once you catch mono, it stays dormant in your system, usually forever. But in my case, I had an extreme case of tonsilitis to the point where the virus got activated again. And then just 2 (or was it 3?) years ago, I actually infected my boyfriend with it, and I wasn't even sick with it at the time.
We couldn't understand how he could have caught it, but I read on the Mayo Clinic website that it is entirely possible to still spread the disease even long after you have recovered and it has been dormant for may years. So I infected the guy when I wasn't even sick. Looking back, it's pretty funny that I did because he was a jackass and a loser and I'm glad i got him sick.