Thanks for all these additional details.
I am sorry to hear about your problems.
Well, such burning sensation in upper left part of thigh, which occurs on few occasions, could be related with compression of the nerves of the thigh, it can be either the Femoral nerve or upper lumbar nerve root.
If you have such symptoms in lateral part or the side of the thigh then it is likely to be a disease called Meralgia Parasthetica. This occurs due to compression of a nerve called the Lateral Cutaneous nerve in the thigh.
It is possible, that lot of hill walking has caused such symptoms.
Usually it is not a serious condition and will usually resolve on its own.
I would not panic for any serious issue like blood clot.You should wear loose clothing and take tylenol or ibuprofen.
You should remain away from lifting any heavy objects.
You can use Heating pads or hot wet towels on affected areas. You can apply it for 15-20 minutes and it can be done 3-4 times a day.
If symptoms doesn't start to improve in next 2 weeks, then you need-
-X ray/ scan
-Nerve conduction studies test.
All the Best!!
If you have any further question then you can ask me.
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