Spondylolysis
Spondylolysis, that's it. It is the reason behind the pain I keep feeling at my back since July, 2007.
There is some more information in the Internet about it, and it seems like I'll have to loose some weight and do more exercise.
Of course the broken pars interarticularis is there, and it won't be back to it's initial state, but hey, it could be worst! :D