The same thing happened to me last spring. I'd been mildly depressed for most of the winter, and then one morning I woke up and felt like I'd been catapulted out of the depression. I did go into hypomania for a couple of weeks, and then things evened out and I've been stable ever since.

I'm using a light box now, which seems to be preventing depression for the most part. I do wonder what this spring will have in store for me though.