a rash that disappears reappeared about 6 hours later.

My son has a rash that appears like hives.
He had it yesterday but it was gone this morning and we sent him to school. Later in the day he had to be picked up from school for this rash, but by the time we saw the doctor it was gone. It reappeared about 6 hours later.
We have tried to isolate what he may have come in contact with or eaten but nothing stands out as a common factor. It happened at school and home as well, so I can't find any common environmental factors. I thought maybe his clothes but he had the same clothes on today when the rash appeared and when it went away. Also, we use hypoallergenic cleaning supplies.
The only common food item for each day was tomato sauce and strawberry. The tomato sauce was eaten 12 hours before the rash's first appearance, and approx 1 hour before the second; and the strawberries at least three hours before any appearance.