My 5mth old woke up yesterday with a rash all o...

My 5mth old woke up yesterday with a rash all over his body - it is small bumps with red patches the size of a fingernail. I took him to the doctor and was advised it was an allergic reaction. The rash is already gone but now I don't know what to do. I gave him pumpkin and potato mixed together for the first time and because he is a pig I gave him the equivalent of about 1/2 to 1 jar. Could it have been this? I have never heard of an allergy to pumpkin but maybe because I didn't start him slowly??? The only other thing I gave him different was nuroffen - he has had pannadol but is teething and it only lasted 2 hrs whereas nuroffen lasted 6 hrs. Any advice would be much appreciated...