This just randomly happens with no stimuli? It doesn't happen after exercise, or when listening to music, or sitting in a certain position, or anything?
I can't find a single thing that could explain this, or even give it a name. Have you ever talked to a doctor about it? I mean, not that it needs to be fixed, or cured, but I wonder if it has something to do with a nerve response or something similar.
You hear a lot about nerve pain that is unexplained but not pleasurable feelings. But could be the same situation of this? I'd speak to your primary care doctor but know this may be hard to talk about. I couldn't find anything either. Do you get an erection during it?