I can't believe some girls would be so inconsiderate that they would go somewhere without their boyfriend, especially on spring break!
I say break up with her right now, and tell her explicitly your reason for doing so. For example:
"I'm so insecure that I am deathly afraid you will cheat on me with all sorts of guys that are infinitely more attractive than me while you'reon vaction, so I have decided to preempt your sluttish actions by breaking up with you immediately, whether you actually cheat on me or not."
After saying something similar to this, she will immediately drop down to her knees and beg forgiveness for her two-timing desires, inviting you to sweep her into the bedroom and make passionate love all night long.
In all seriousness, she may cheat and she may not cheat. Since you have no way to really tell, simply assume that because you are so attractive and suave she won't, then you won't have anything to worry about. After all, she's just a girl.