Hamstring pain

If my right knee bends a certain way (usually 45 degrees or less) then my hamstring goes into a spasm/cramp which is extremely painful but disappears on extension. I play soccer but only ever feel the hamstring at absolute top-speed. I know the obvious solution is to avoid bending the knee that certain way, but sometimes it cannot be helped. Is this a sign of a hamstring strain (there’s no swelling or bruising) and are there any stretches which would help?? Although this is more of a hindrance than an injury, any help would be greatly appreciated.