knee to chest tightness

[Eating The Bear]

just doing a simple knee to chest stretch while laying on my back i get some unusual resistance.

i'd expect it coming from my hams/glutes/lower back but i feel like my upper quad is actually pushing my leg away from my torso. i pull my knees to my chest and it almost locks before getting there with the area i've marked in the picture doing the locking. i have no problems doing a quad stretch such as pulling my foot to my butt while standing.

i'm 150 lbs/5'11.

the resistance comes from the area i marked with the arrow.
http://img51.imageshack.us/img51/6748/quads.png

any ideas on what causes this or what i can work on to help it?

thanks in advance.