You don't say where you are burning. Is it on your back or all over? Have you changed your detergent or soap lately? Did you change your fabric softener?
It sounds like an allergic reaction to something since you had all those tests and they came back negative.
You might try going back to what you were using before - or try something new that contains less chemicals. There are some new "Green" products at Walmart that cost about the same as the discount ones.
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