It could perhaps be chlorin, I swim a lot & usually it's fine but I have noticed slight rash on my back once in awhile after swimming.
I've also thought it could be the perfumes they use in the shampoos, as most the ones that bother me are the ones that smell really strong.
As for shampoo brands, the most reliable ones have been the ones that I've bought in healthfood stores.
Here are a couple of brands
(To moderators, I hope it's ok to list a couple of brands to help other people with shampoo allergies):
Nature's Gate (I REALLY love this one, I usually get the Aloe Vera one or Rainwater, but I'm sure they're all good)
JASON (the A has a curl above it and the O has two dots)
I also belive "Burt's Bees" has a shampoo that I used once & it didn't bother me, but the two above I've been 100% happy with.
I'm sure there are other herbal shampoos that are good, just check your local farmer's market, trader joes or other stores around you that sell natural products & herbal products