Bumps, help?

Hello, This is my dog Quincy! I’m very worried about him and I’m scheduling a vet visit as soon as I can, I’m just wondering if anyone has any clue what is going on. We started off thinking that he had heat rash, as our other dog had it first and then appeared on him. (We hit the massive heat wave up to 116 in OR!) but now it has spread to his fur. It got really bad really fast and now he has many dots on him. I’m worried about if I should keep him away from my other dogs? Or if there’s anything I can do for him? 

Hello!  Where in the world are you?  I will answer as though your are in the UK, but there may be infectious diseases to consider in any country. Skin problems such as this develop fast so we agree that it's best to see your vet, even if it seems to go down again.  However, there are lots of possibilities. First, it is striking that two dogs are affected; one and then the other in quick succession.  It would be unusual for two dogs to have the same allergy, so unless they are closely related, it would makes one think of particular irritants.  One common irritant that appears in two dogs in the same household, is fleas.  Fleas, contrary to popular belief, are not even always noticed and many dogs showing flea reactions don't have a single flea on them; a sensitive dog only has to go 'relatively near' to a flea to end up in this state. Insect bites, such as midges, may be another possibility. One of the pictures looks like urticaria ('hives'), an allergic reaction that causes raised, hairy lumps a little like coins, which go down again often relatively quickly.  This could be in reponse to a flea or a plant or something else.  I have seen it happen convincingly in response to ivy, for example. Our recommendation would be to call your emergency vets, particularly if the condition is still developing (getting worse). The may recommend certain drugs to help, or they may triage and to check whether or not you are safe to wait until morning to be seen.