Tightness in finger joint

[quietnoise]

Hi,
Recently I have started to notice a feeling of tightness in the 1st joint and also knuckle of my middle finger of my left hand. The feeling is very fleeting, lasting only a second or so, and there is no pain whatsoever. It isn't happening very often, and happens both when I'm playing and when I'm away from the piano.
I haven't been able to get an explanation of what this is; it has been suggested that it could be trigger finger but the symptoms don't appear to match. It doesn't feel like a spasm, it feels like somebody is grabbing the finger joint and letting go.
As I said there is no sensation of pain whatsoever.
I haven't read of anything that could explain the problem, and I was wondering if anybody has experience of something similar, or could point me in the right direction?
Obviously I am very concerned and do not want to make the problem worse, having suffered bad RSI in my wrist from guitar playing in my teens.

Thanks

Mark