I recommend you go and see a dermatologist as sunburn does not spread from one leg to the other - so either you damaged your skin and now have a condition that has spread or the reason you burnt there in the first place is because you had a weakness there before hand.

It is possible that you have an allergy to the sun - which is more common than it sounds - they will be able to tell you.  I get a heat rash on my chest if i sit in the sun without putting on a special sun allergy cream first.

good luck