I was doing deadlifts and felt and heard a loud pop while bent forward. this was followed by pain, some numbness, and loss of mobility. it seems to be where the inner hamstring connects to the sitbone. it is only acute when extending the hamstring?

Tuberosity: The hamstring which consists of the semimembranous muscle and semitendinois orginates at the pelvic bone on your ischial tuberosity. Likely you have strained or even possibly avulsed the tendon. Your sciatic nerve runs through the area and that is what likely caused the numbeness and pain. In general this should heal in a a week or two.