Having trouble refocusing my eyes?!

I've always had times-even when I was little-that my eyes would unfocus on their own. I would wait a few seconds and it would return to normal. Until recently-a year ago-I didn't know that I could do it on purpose. Every once in awhile, if I'm bored or something, I'll unfocus my eyes and then refocus them.
I was doing that for a longer period of time today, and now my eyes are continually unfocusing and it takes an effort to refocus. Then it only stays that way for ten seconds at most, and I do it all over again. I have pretty good vision and I really don't know why this is happening. Help?