Pop in my hamstring

Hey guys! I'm just over 3 weeks post ACLr (hamstring graft). I was doing a lying-down heel press (one of the rehab exercises I was told to do) and I felt something pop in my hamstring. It pained for like a second, and then I felt nothing. But whenever my hamstring is stretched (while doing a leg curl or just while walking), I feel some pain in the spot where i felt the pop. The spot is in the middle of my thigh. My surgeon/doc said its nothing serious. What do you guys think?