Strange rash

Small bumps suddenly appeared starting with one bump then spreading in little welps or clusters first on my neck and shoulders, arms and back -  after soaking in epsom salt and applying Sarna they cleared up on my upper body but the next day broke out on my thighs,  but once again, they went away after a few hours.

Throughout the day, I am seeing one or two bumps return on my arms, but not as intense as the first outbreak.  

I thought this might be poison ivy, but wouldnt poison ivy take a few days to treat and disappear?  Do you have any idea what this might be?