"Impossible to get an appointment "

My son is 3 and had his mmr injection last week. For five days now he has had a high temp, fever and yesterday broke out in a rash. He hasn't slept for four days and is scratching at his skin. I have been giving him calpol every four hours. I have tried today and yesterday to get him an appointment, for the receptionist to first ask for the nature of his problem then deem him not poorly enough to see a doctor and tells me there are no appointments at either surgery with a doctor or a nurse. She advises us to go to the walk in center where there is over a two hour wait. My son cannot sit in there for that length of time feeling this unwell. When he was given the injection we were told if he had any side effects to see a doctor this is impossible. I wish I had never allowed him to have the injection and I see why so many people decide not to when the after care is so poor.