eating less, moving more; GAINING WEIGHT?

I started running a bit more mileage after my track season ended, to prepare for cross country (from about 30 to 38.5). I've been running anywhere from 5-7 miles in a day, lifting weights 2-3 times a week (usually 3, although i've been skimping out on leg work lately; changed that this week however). I didn't eat too much differently once summer started. I mean, I would splurge at parties and stuff, but I would do that during the year too. And when I did, I usually wouldn't be very hungry the next day. So, one day, i'd have like 2800 for example, and the next i'd have 2000 because I just wasn't as hungry.

But, the scale's gone WAY up (from 108 to a solid 112) in the matter of a month. I doubt it's water weight. I don't understand how this happened! I'm eating just about the same (I started trying to eat more healthily again, so i'm eating less than I was during track at this point most likely) and I weigh MORE! I'm still at a healthy weight, but not necessarily where i'd like to be. Why did this happen? How do I stop (or even reverse) the gain?! It just confuses me so much and I need some help. Thanks!

EDIT: I remember this time last year maintaining on 2500 or so no problem. I was 16 then, 17 now; did my metabolism slow down THAT much in one year? It's really depressing if it did, cause that means i'm now gaining on 2200-2400 calories...

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