Has anyones toddlers developed hives out of nowhere?

Hello,
Our sweet little 3 year old just started to develop hives out of nowhere...theu come on for 5-10 mins and then fade.  When it first happened we cleaned everything and then it continued so we have been going over in our heads if it is her diet...she is an extremely picky eater...or allergins. Or something as somple as oreos. We are just worried and trying to figure it out. Anything will help.