slightly painful errection

On a every so often occasion (maybe once a month to 2 months) ill awake to a erection thats so hard its to the point of painful (generally on the upper shaft left side), not horribly painful, still hurts but more of a large discomfort,  that will not go away, until i *fix* it. then within a few minutes will finally dwindle back down. What are possible causes to this?

[NOHARD]

Hi Matt You have no problems its called a nocturnal wood, we all get them and yesthey do wake us up sometimes, its best to just get up and go fro a drink and by the time you get back to bed its going down, unless you have a wife then she could be on the receving end of it.
Good Luck