the only thing i know when you lose AP is when you send a message in their profile and deleted it. I did it the same. i deleted all of my friend's messages. Then, my friend said that they loseAP by deleted all of the messages.
You also lose AP if you made spam comments in games or on the forum and they were deleted. The best way to not lose AP is to make sure each post you make is not spam, and has something constructive to say.
Yes, like they said above, someone either deleted one of your comments on your profile. Or one of the mods deleted some of your spam comments/forum posts in their never ending quest to destroy spam! But ya, it's one of those two things.
