Knee giving way...but no pain?

Shmaggle

Help...little half hour run on the treadmill, squeezed into a very busy day....and my right knee kept giving way! I didn't fall over fully but sort of lost the stability in my knee. Not painful but I do now have some vague pulsing sensations in it, underneath the knee cap at the front....

I do wonder if I had carried on for longer it could have given way fully, it seemed to be increasing to every 3/4 minutes by the end of the session.

Any ideas?! Anyone else had this?