Geez, is there any way you could explain it to your boss? I hate when people jump into conclusions like that. And what the f is wrong with your girlfriend? I understand there's never a good time to break up, but during work?? Looks to me like she's scared of what your reaction would be so she did it in the one place where you're supposed to not show your emotions.
Breaking up should be a private event between two people. Do it in a private place, for Pete's sake.. Now that I'm done with my rant, Stay strong OP!