Hey, I'm 26 and I noticed the same thing. There was a time when it was insanely easy for me to drop 5 or 10 pounds. Now, it seems like it's only easy to gain it!
Kickstarting your metabolism is going to be different for everyone, but what worked for me was starting to exercise and moving to more natural foods (less preservatives, less bleached flour, less sugar, etc).
I'd keep working out and keep watching your calories, but start trying different things every couple of weeks until you see the scale start moving again. Good luck, and keep us updated!