A real pain in the ass!
Proximal hamstring tendinopathy is pain felt in the tendon that attaches the hamstring to the sit bones. Often, people report pain during and after sitting, walking, and running. Sometimes, pain can move from the back of the thigh to the back of the knee.
It can be a result of over use or increasing training load too quickly. Or simply just weak tendons.

After an initial period of active rest - try to avoid complete rest as tendons do not like rest - gradual loading can begin. Tendons love loading. A Biokineticist or physio can be very helpful during your rehab to ensure steady progression. Stretching the tendon should be avoided as this will just increase pain and inflammation.

Pain can be managed by the use of cushions; try not to sit on any hard surfaces. Use a small rolled up towel or cushion when driving to take pressure off your sit bone/tendon/nerve. Neural flossing can help if you experience nerve pain.
Shockwave therapy and EMTT have also been successfully used to treat tendinopathies.

Recovery can take from 4 weeks to 6 months!- patience and rehab dedication are key.

Apparently high hamstring tendinopathy is becoming quite common in female endurance athletes over 40 and quite a lot of research is being done on it.