Eczema and possible triggers/allergy

[diveitch]

My daughter is 2 and has eczema which has been treated via dermatology. This was treated as genetic eczema as I also suffered as a child. However, in the last few weeks she has flared up twice after eating lentil soup - face/eyes red and blotchy, very itchy under chin although blotchy all over body too, she has then also suffered with diahorrea. Could this be a food allergy? Or just a coincidence?
She was similar after trying prawns once before (not tried again).

I know dermatology mentioned not testing allergies on children due to false positives, what age would you recommend tests are more accurate.

Thanks