See a doctor, it could be body lice.
I had the same thing happen to me in late July/early Aug I started see small red spots all over by arms, shoulders, chest, sides and lower thighs. After 3 months I stayed up all night and found a tiny bug on my upper arm. It was less than 1mm. I put it in a plastic bag and a pest control outfit looked at it under a microscope and identified it as a body louse.
My doctor prescribed Permethrin cream and I am currently throwing away any clothes I dont need, and washing the rest in HOT water and HIGH HEAT dry and keeping them in plastic bags until this can be resolved.
It *****. It really *****.
Hi
If this has happened before and this happens 100% each time you take a shower, then this may not be something urgent. This may be a chronic skin condition or probably an allergic reaction to chemicals found in the water. If this happened just recently or if there are any associated changes in the choice of lotion or body wash, then an allergic reaction may be likely. This requires further evaluation. Although the rash disappears after taking a bath, a referral or consult with an internist may be able to help for a complete workup.
Try to gradually modify water temperatures when you shower. Try to set is at lukewarm temperature. Do not use extremely cold or hot temperatures. If you are fair skinned dilation or constriction of blood vessels may readily color the skin and may even appear as mottling or as a rash.