Tomatoes?

I know it's not going to be terribly productive to start blindly eliminating more foods but I'm having an overwhelming tomato suspicion at the moment.  Over a year ago Roo wasn't eating much tomato...perhaps here and there.  That could explain the "clean" scope.  These days he's a regular ketchup lover and has recently taken to those luscious little rubies otherwise known as grape tomatoes.  

Tonight, while dipping his fingers into ketchup and licking his fingers, I swear he got a little rash on each cheek.  It was slight and Mufasa wasn't here to verify, but it was there.  And it hadn't been there a minute ago.  It eventually went away but then he threw up tonight after dinner.  This one seemed like a clear reaction to something...no oral delay or gag to speak of.  So, I'm left wondering.  Maybe it's the tomatoes.  Then again, patch testing to tomatoes was negative so...ugh.