I've had this exact same thing happen with a very similar looking rash. In my case, I kayaked for like two weeks without washing the shirt, probably went like 5 times. I suspect that I picked up some kind of fungal rash, like athlete's foot, except on my waist and back where the spray skirt holds the shirt in contact. Also I almost never wash my spray skirt (do yall???). The rash spread mildly too my arms, in the exact spot where my arms lay over the rash area when typing.
I'm trying to treat it with cream for athlete's foot currently. Will try the steroid cream next.