I experienced a sudden flare-up a few months ago. I normally have pain in my legs and feet, weakness, balance issue, etc. but his was a body wide burning feeling, like a really bad sunburn. My clothes were unbearably painful, sitting, everything burned. I first suspected my meds, so we changed a few things with no effect. A few weeks into his I developed the biggest, ugliest cold sore I ever had, and it took weeks to heal. It wasn't as painful as I remember them being, but gabapentin is very effective for post-herpetic neuralgia, so dampened the pain.
It now seems likely that a shingles type virus was responsible for the burning. Once the blister broke out the symptoms gradually lessened, and I am back to my normal level of skin sensitivity, which is still elevated, but tolerable.
My doctor has given me a prescription for an acyclovir type medication to take immediately if the symptoms return, or I develop another cold sore. If a virus can cause a flare, I would guess an infection could as well.