^ Very important point.

It's appalling how alignment and posture and range-of-motion issues in the pelvic region can impact many of the other muscles in the area, along with knees, back. "Alignment" is a big issue, in a sense. Not the alignment per se, at least in my own experience, but what poor alignment suggests is going wrong in certain areas ... whether weak ab/oblique and/or low-back muscles, tight piriformis and general glutes, weak flexors and stabilizers around the legs. Hard, for the big leg muscles to do the job they're designed to do if the pelvis stability just isn't there due to some other muscle(s) being too darned tight. Longstanding leg injury, here, so I've been dealing with a wide-ranging strengthening and stretching regimen for decades. Works, when it's taken as a whole. Can often fail miserably, if only imagined it's a "stretching issue" that stretching of one muscle will cure.

In my own case, very tight flexors and stabilizers cause poor pelvic alignment, which impede the glutes and hams doing their job, which tightens up the piriformis and hams, which begins to ache in the low back from all the wonky stresses, ... But, fix the tight flexors, and two-thirds of that simply disappears; strengthening the abs/obliques/back and stretching the piriformis more deeply, fixes 98% of the rest. But, that's me.



Definitely take the time to see a knowledgeable physical therapist who's well aware of sports-related injuries and training, specifically competent in the pelvic/leg areas. You might be surprised what's involved to actually get to the root causes of your issue.

Ditto on the book that was mentioned. It's a reasonably good one, for including a wide range of illustrations on useful stretches all in one book: Stretching, by Bob Anderson. I keep the index to this book with me in my gym bag, in my car (for trips), in the luggage for traveling. Good basic reference.

Another pretty good one that'll help with understanding the muscles through visualizing which areas of which muscles get hit with which stretches: Stretching Anatomy, by Nelson & Kokkonen.

Paired with good info from a competent sports PT, you should have what's needed to correct the issue and keep ahead of it.