Sounds like they are tight.
I had the same feeling all summer.. They were so tight, when I was sprinting, it honestly felt like they could snap. It was uncomfortable as hell (and somewhat scary). The weird this was I stretched them ALL the time.
BUT never lying down with a straight leg to ensure my back was straight. My hammies were tight up near my butt as well, and it was tough to stretch those out.. ZiR pointed me in the right direction:
make sure to keep you back flat and legs straight... see if that helps.