If it's clearly at the tops of the back of your leg, forward and below your butt cheeks, and it feels like you're sitting on a golf ball, or something similar, suspect piriformis syndrome. I linked one site with more info, but there's plenty more available with a simple search.

The bad news is that it can be with you for life, but the good news is that it's easily managed and can be 100% resolved with some basic and simple exercises. Once resolved, it can be gone for months, but there's high likelihood that it'll come back and you'll need to deal with again.

The best news is that if it is piriformis syndrome, it's more annoying than a serious health issue, so use the info to diagnose and resolve it with the specific stretches.