It sounds like you may have a larger tear in the muscle or musculotendious juction somewhere. I would try to get either a diagnostic ultrasound on the muscle (perhaps dynamically to assess the fibres as you perform a hamstring curl with the weight for the leg only) or an Wikipedia reference-linkMRI. Stretching is not going to assist at this stage. Nor are hamstring curls in the gym. You must consider an investigation at this stage to ascertain what is damaged and the extent of the damage.