when I had shingles it was just a slightly sore eye. It comes in different strengths. But it only appears on one side of the body, so that ought to rule out anything going right across your back.
But it could have been a bite. I have several big red itchy blotches on my arm and stomach from when I was weeding the lawn. I never saw what did it, even when I looked, but I had factor something clothes on and it was cloudy so highly unlikely to have been sunburn. I don't know if this was true of you, though