That's weird.Are you sure you don't have a tension in your hand? Is this thing happen when you play really slowly? Video could be helpful, so please post it.
I went to a physiotherapist, and he said that my wrist got blocked, after some too heavy practicing. Because of that, my lower arm started over-compensating, and eventually went too far.He tried to convince me that it would be fine, and that it was just a matter of time...
The thing that's confusing is that I don't feel any pain, at all...
It doesn't sound like nothing to worry about. As it's confined to between two fingers, I would suspect that you've damaged specific tissue in that area. I still think it would be a good idea to have an x-ray or ultrasound done to investigate and see if an actual physical problem can be established: unfortunately these things cost money and your health service may be reluctant to spend it.