Scarpozzi, do you know any women at all? I only ask because you don't seem to be getting why she is upset. Whether or not you were on a "break" (quite possibly the dumbest, junior high drama crap ever, btw), your relationship of 1 year and 7 months was obviously unimportant enough to you for a couple of days apart to result in you making out with some other random girl. Girls (and guys) tend to get mad when their ex hooks up with someone else a day after the breakup, right? It's the same concept, people (especially girls) like to think relationships were important enough to the other person that they don't "get over it" that quickly.
Look, I'm not saying I agree with that logic all the time, but you've got to admit, when a 3 or 4 day break is long enough for you to end up making out with another girl, that sends the message you aren't too interested in your relationship. THAT'S why she's upset...I'm not sure why you didn't see that coming.