Twig, is it definitely an allergic reaction? Dd gets this red patch thing from eating strawberries, being snotty, having a dummy (trying to do only at night and for sleeps now though) and general slobber. Could it just be from eating?
Twigglet my DD2 has the same thing, can you give her Priton, My DD2 regularly goes to bed with a bright red mouth, I would keep an eye on it, and if you plan to give her the MMR (sorry to bring it up!!!) make sure you mention it to HV.
I try and keep eegs away from DD2 as she is always coming in red rashes and hives but she loves them and likes to finish off my fried breakfast when DH makes them, I give her Priton and it goes, we have also discovered she is allergy to red colouring, in beans sauce, smarties etc.