I dont have any suggestions as to why it happens in the summer, but not winter, but I wanted to let you know that this happened to Violet once after a bath. The hives came so quickly and they were hives on top of hives and her entire body was covered in layers of hives. I took her to the ER and they had never seen anything like it. I requested and Benedryl and antibiotic shot. The gave it to her and told me "I dont think it will help b/c it doesnt appear to be an allergic reaction" Surely, it didnt help. So the following week I took her to the regular vet who did a skin biopsy and it came back a "drug toxicity". Odd b/c she was not on any drug but her thyroid meds, and I KNOW she didnt get into anything toxic either.
This did happen in the spring time (though I never considered the time of year having anything to do with it). And the vet maintains that it was a drug toxicity. So, the only way I can think of how that would happen is that I tended to clean the dogs the same day I cleaned the house and maybe having contact with the house cleaners caused the toxicity. The vet didnt seem to think it was related to the bath, but I disagree b/c with in a couple of hours of the bath, she was a hivey mess. Whatever. I switched my household cleaning products to "green cleaners" and only wash the dog in gentle non-soap shampoos and have never had the problem since. The other thing is that the ER vet said it COULD BE (but no one really knows) that because I "scrubbed her really good" that day and I could have caused perforations in the hair follicles. So, that is another thing I do differntly, I scrub her VERY gently during bath time.
It was never determined what happened exactly, I only know she had a drug toxicity which was impossible, so, I made a few changes and it never happened again.
Good Luck.