You aren’t heel striking, but how are you striking the ground?
I’ve see this happen when someone is landing on their toe vs mid foot.
Would be a good idea to have someone video tape you from the side so you can see exactly what is going on.
Couple ways to hit the adductor and high hamstring
Episode 14: Celebrity Goat Death Match: Chris Spealer
Pro Episode # 36 – Voodoo Floss Series # 3: The High Hamstring