The same thing happened with our DS except he didn't have any PB so we couldn't trace the cause.

We took him to several Dr's and one asked us if we had recently started daycare or playgroup or if our DS had recently started having contact with other children. And the answer was yes we had just started playgroup. Apparently some children react to coming in contact with viruses by getting hives. He told us that eventually it would stop as DS immune system developed a bit more resistance to things and he was right. A few months after starting playgroup no more hives. They weren't itchy, didn't seem to bother him just looked awful and were very red. They'd come out of know where and then fade away. It was one of lifes little mysteries!