It might be possible it was from the PB but from what i have read and been told food allergies are usually instant..

with my sons hives they think it might have been some sort of plant on the beach but he had the same reaction twice more in different places.. My eldest well we have no idea what caused them but they were huge.. and he said itchy..

I would avoid PB around him completely though for a little while and see how he goes.. Is there anything else at all.. New detergents? new clothes ect...