Dayna,
Have your pediatrician recommend a pediatric allergist.
I have one son I have always dealt with allergies/asthma since birth. He is allergic to every tree and grass pollen, smoke, molds, anything environmental of those sorts. My 11 yo son has recently shown signs of allergies with constant congestion, cough, sneezing, stomach miseries and we had him tested. Found out he is allergic to oak pollen - no other environmental allergens or pet allergens. He is however sensitive to dairy, wheat and peanuts. A 3rd son has very sensitive skin and has to stay away from things like citric fruits, katsup etc., but only to the extent it can't touch his skin.
Bottom line, you cannot normally guess your way through allergies. I encourage you to take your child to an allergist if symptoms persist. Allergies make you miserable and can really affect the behavior of many children. And often if you can pinpoint the allergens, you can avoid them and thus avoid any medications.
Good luck. I totally empathize.