From your description it sounds like the Biceps Femoris which i think is where most hamstring injuries occur. The hamstring group is made up of an additional two "semi" muscles which help with the medial/internal rotation (towards body) so if this doesn’t cause any additional discomfort then it might be further proof that the Biceps Femoris is the culprit.

Either way use RICE and you’ll be good in no time.