My son had the same thing happen to him. I sprayed his room down, sterilized everything, called the exterminator, had the carpets cleaned...He still continued to get them for a while afterwards then it just stopped happening as quickly as it started. He is 2 now and I can't remember the last time it happened. I think he was allergic to something like the laundry detergent, dryer sheets...or something that we were feeding him. It could be some kind of bed bugs, but I would think that would be unlikely given the fact that you have washed everything. Also hives, which come from allergies come and go quickly like you described..."gone within 5 minutes". I hope everything is ok...give your pediatrician a call on Monday, that is what I would do.