As it's only on one side and always in the same place I am unsure what it might be.
I used to have skin sensitivity and photosensitivity and this turned out to be at least partly due to a vitamin D deficiency. It would be worth getting this tested as around 50% of the population don't have high enough levels of vitamin D. Mine was generalised sensitivity though, not localised, so this may have nothing to do with your symptoms but it is worth getting it checked anyway.
It could also be some sort of nerve thing. Where you are describing sounds like the sciatic nerve. It could be that something is impinging on this nerve. Have you ever had trouble with your lower back/hip area? This could be causing the nerve to be pinched.
It is also important to know that B12 is important for nerve health so this is another test that is worth doing, even though it might not be directly related to your symptoms, and if the result is below 500 you might want to consider taking a sizeable dose B12 supplement.
Those are just a few ideas, but unfortunately I'm not a doctor so I could be well off the mark. I do suspect the sciatic nerve issue though as it is localised to that area.
Hope that gives you a starting point at least.