Sorry my anantomy knowledge is not the best. I'd say it's my Biceps Femoris based on the location of the discomfort.
Height: 5' 11"
Back Squat: 1RM 325lbs
Romanian Deadlift: 1RM ~220lbs (I've never tested it, I only use it for assistance work)
Leg Curl: 1 leg: 1RM ~90lbs (I've never tested it, I only use it for assistance work)
Deadlift: 1RM 315lbs
I currently follow CT's HP Mass Program and am beginning week 6. Prior to that I had followed Jim Wendler's 5/3/1 for about 4 months. Our coaches put some good focus on sprint technique in high school football, but that was 15 years ago, although I'm sure I've maintained some of the technique. Let's say it's average and room for improvement.
Flexibility: Not sure how to judge my ankle's flexibility, it seems good. My hip's flexibility could improve and I think some of the frequent Squat work has somewhat reduced my hip mobility.
IT might also be do to a pre-existing injury. About 11 years ago I pulled my hamstring really badly playing football on an astroturf field. I could barely even walk for the following two weeks and I couldn't squat again for almost six-months. I probably should've seen a doctor and if didn't happen the week before I was getting married I probably would've. It may not have healed right which could be a problem.