Maybe you're lonely. Hormones are still a mood-swingy factor, too. Be sure you're not surrounding yourself with company that might be dragging you a bit too.
It can be anything...but at the core of it, I recommend meditation. Just chillin', by yourself, no noise, no distraction, and literally making an effort to completely clear your mind. Hit the reset button on things...then, reflect. Identify what you could change or what you can do to improve whatever you need.
Thoughts = Behaviors = Results
You don't like the results, changing behaviors won't do shit...the same thoughts will eventually cause those same behaviors to resurface. Start with how you think. You are the captain of your own ship, and that's really all there is to it.
I wouldn't fret. Everyone gets depressed every once and a while...you did good to vent it out! Now do somethin' about it.