Thanks for all the replies. It is probably from a host of factors- bad technique being one of them, I'm sure.
I saw a sports doc. He had a look at it and played with my knee a bunch and made me flex and push at different angles. His conclusion was mild tendinitis. He said it was the kind normally caused by impact. For example when stepping down from up high and the quad/leg adsorbing the shock on landing.
This surprised me as I couldn't think of anything I had done lately that could have brought this about. Just one of those things I guess...
His exercise/rehab recommendation was to get into air squat position and let my body 'fall down' and then 'catch/stop' the drop just before I felt the pain. So instead of a controlled squat, I would let gravity take over, maintaining proper form of course. I don't know if that is a clear description or not...
It has been a while now and I can do full air squats painlessly again. However I have been doing sets of five just to be careful. I used to do sets of ten pistols
I will be seeing him again tomorrow to check up on some other things. I will bring up shin angle and see what he says.
Kind of turned me off squatting though. Maybe I will try high bar or low bar once I'm healed.