Top of my left leg is sore - any ideas?

Basicaly, when I've been running uphill, or been for a longer run, or not run for a while, I get a soreness in front of my left hip. It isn't in the hip, it's more superficial than that - right at the top of my leg on the front, outer part of my thigh (if that makes sense). It only happens when I run (and not always if I'm on the flat) and doesn't bother me at any other time. It doesn't stop me running, although if I walk and put a hand over where the pain is, I can feel a clicking as if something is getting caught on something else. I can't feel the clicking if I don't have the pain.

 Anyone got any ideas what it could be and what I can do about it? I do have proper shoes btw.