Scary reaction

We were at a friend's home for a birthday party with the kids.  We swam, played outside, ate hamburgers and cake and watched her opened presents.  Hudson suddenly began itching and his left eye swelled up.  Then he broke out in red hives.  We thought it was ant bites at first but he had large red whelps all over and his eye was getting bigger.  Then he began coughing.  We gave him some Benadryl and Zyrtec and quickly drove home.  By the time we got home he was coughing and wheezing pretty badly.  Thank the Lord, I always keep extra children's oral steroids in our cabinet.  Lord knows it has saved us many a trip to the ER with our kids breathing issues.  We stuck him in a cool bath and gave him a breathing treatment.  What a blessing to have modern medicine.  He cleared up and was 100percent better!!  Whew, another trip to the children's hospital avoided and best of all he was fine.  But, the scariest part is that we have no clue what he  had an allergic reaction to.  I was thinking a plant or bug, but Mike thinks it was an ingredient in the cake since that was what he ate right before the reaction.  How do we know what to avoid?  Everything we ate tonight he's had before unless there was something new in the cake.  I guess I'll get the recipe and investigate.  Hmmm, so do I carry Benadryl with me all the time?  Poor sweet baby boy.