It’s IT. The Illiotibial Band runs along the outside of your thigh, from the top of your hip (the iliac crest, that bony ridge) and down to your knee. With runners the IT Band is a common malady and I’ll go out on a limb and say that yours is probably tight unless you’ve been working on it.
What does ‘working on it’ mean? Diligent stretching (we all know how good I am at that!) and breaking up the knots. One of the best ways to focus on the latter is to use the foam roller; I’m going to warn you that if you haven’t done this before it’s going to be pretty uncomfortable and maybe even make you want to curl up and cry…so bring a tissue. 😉