Now that is an odd one I've never heard of to be honest. Hm. Heat rash? I'm not sure what kind of correlation there would be with bumps down your arm. Are you positive they are never there otherwise? There is something called 'stress flush' after orgasm. Maybe it is somehow involved. If you have a well check doctors visit, and you are feeling like you an talk about this in person with somebody, you might get a doctors input. I'd love to know what they say! How long do the bumps last after and is it one arm or both?