I started Muay Thai today, and when doing a basic kick (a straight teep) above my waist level, I felt strong resistance/mild pain where my leg connects to my hip, right at the top of the leg, by the joint. This sensation was in the front of the hip, at the top of the quad.
It wasn't the kind of pain that's associated with injury or anything like that-- it was distinctly the "this doesn't stretch that way" feeling.
But...I consider myself flexible. I can do a longitudinal split! However, trying a sideways split feels like I'm going to break a hip.
So why can't I kick above waist level? And how can I improve this, if indeed I can?