Allergic Reaction

So my son is 15 months and today he broke out in hives and than threw up a bunch. I took him into the walk in and he's now on Benadryl. He was around a long haired cat today does any parents out there have a child who is allergic to cats!?! what happens I just wanna know why my baby is breaking out I feel awful. Also I just found out my grandma gave him home made ice cream with cashews in it... I just don't know enough about allergic reactions nut relations or pet reactions to really know what's going on. I'm scares nervous and over whelmed and just want my baby to be happy and healthy..

PS it was also in his hair line bright red rash .