@letshearit - I also used to get semimembranosus pain and found that certain stretches and rubbing the area with a muscle soothing cream really helped.
The best semimembranosus stretches I have found are the sitting hamstring stretch, the standing hamstring stretch and the partner hamstring stretch. All of these are easy to do and you can find images for instruction if you search online.
As far as strengthening exercises go I would try using the knee curl machine at the gym and doing some hip extension exercises with a resistance band whenever you are at the gym. You'll feel a lot better and the pain should go away once you strengthen up.