Yes, it is my plant/stride leg. If it were a strain or pull wouldnt I feel it even when I am not pitching though? I mean, I can pitch, feel that pain, then stop pitching and sprint and it wont hurt one bit.
Flexibility certainly could be an issue. I am out of playing shape. I currently stand 6'2", 192 lbs. and turned 17 in March. I dont appear overweight but my body fat % is definitly over 20% and I am not very muscular (although my legs are quite strong from a decade of soccer.) That is another thing, I throw easily mid-70s right now, and I really think with some fat-to-muscle conversion and general weight loss I could be throwing in the 80s.