What Causes Recurring Dull Pain In Back Of Left Thigh?

I have a dull pain in the back of my left thigh. It has been occurring for several months and I can not attribute it to any one activity. It occurs often, several times per week...not extremely painful but starting to worry me since it happens often I drive about 400 miles a week for my job but try to walk 2-3 miles on the days that I drive a lot. Any ideas? 45 yr old female in fairly good shape and medical health