This sounds a LOT like a condition called:
ILIOTIBIAL BAND SYNDROME [ITBS].
ITBS is an inflammatory condition involving tissue called the Iliotibial band.
These links explain ITBS in much more [and accurate] detail, including what you should do to treat it. After reviewing, feel free to reply back with a followup question.
Hope this helps--take care,
Here's link from MayoClinic.com :
So--the AREA of your symptoms IS consistent with ITBS. It doesn't HAVE to occur with strenuous activity: that is just what they call "textbook cases". You have to remember--there are a variety of possible symptom-presentations. The exact TYPE/degree/nature of pain can vary as well.
[the treatment for these are very similar...including R.I.C.E. that's mentioned in the links I gave; as well as Physical Therapy].
I still think ITBS is the most likely...I have managed a number of cases in my practice--and a few had IDENTICAL presentations as yours; again, there is no ONE precise presentation--there is a wide variety. Of course, there is no real substitute for seeing your doctor in person, to get a detailed exam done.
Hope this helps,