orange rash

hi

My son is 15 weeks old and for the past 2 weeks he has been coming out in a rash that last 20 mins and then disappears. It mainly starts on his foot then spreads up his leg and sometimes throughout his body. His foot and part of his leg turns purple with a bright orange rash also appearing. on the worst occasion his foot turned purple with the orange rash spreading upto to his knee, then his foot became swollen again within 20 mins the rash disappeared and the swelling went. In the past week everytime this appears he starts to scream asthough he is in pain.

We have taken him to the gp and A and E on seperate occasions to be told nothing can be done until they see the rash but unfortunately only having 20 mins before the rash disappears we never quite manage to make it on time for someone to see it.

Any help would be greatly appreciated

many thanks