It would be a possibility thats for sure. Just before you give him any more baths I would really rinse him off well, even good shampoos can cause irritation if not rinsed thoroughly. But you said he "wasn't this bad until I gave him a bath" does that mean he has some slight irritation? If so then the shampoo probably caused the outbreak, weather it was actually the ingredients or just the irritation of being bathed.
Personally I'd rather be safe than sorry & dump the shampoo.

Good luck at the vets & let us know how it goes!