I've experienced the same problem. It started this year. I think it has something to do with the dogs being in the pool. The dogs might be washed
with special anti-insect shampoo which can cause genetic flactuations in the dogs treated. That's why only vets can prescribe it and only to dogs that
are 'fixed'.

I do not think chlorine is the problem. Most of the commercial pools are
chlorinated and allergic reaction to chlorine is redness all over the exposed
body parts.