Answering my own question - it is called "neurotension" and he gave me an exercise to address it. I could actually feel my leg loosening up as I did it

I have been given the go-ahead to do very light spinning with little resistance in spin class, and orders to put it off another week if it hurts at all. 1-2 more weeks before I can ride my actual bike. I think I will hold off on clipping in for a few weeks so my hamstring doesn't have to work so much.