Has he got any other signs of being ill? high temp? off food? affected by bright lights?
Their records may well state if hes had MMR and measles is still (luckily) pretty rare. Measles is also preceeded by very high fever as are many of the rash type illnesses.
My son has had viral type rashes a few times and the GP just send us on our way.
If he is still covered in spots in a few days, or if he has a fever which you cant control or any other symptoms then take him to your GP or call NHS direct again for advice.
oh and of course youre not being over anxious, thats what all mummies are like! :)