I agree with Jen, it may be from the shampoo then, since you have stopped it.
i doubt it is related to teething. The red cheeks and diaper rash may be, but not this hives.
It really could be a mosquito bite or some sort of gnat. I know when we were at the beach last year, my son got bite by "some" kind of flying bug and got hives like that. They just kept popping up. I remember taking him to the allergist wondering what was going on.
Once they clear up, you could always try the shampoo again and if they come back, then you have your answer. Then you would know for sure.
Hugs to you, Wendy