Scary allergic reaction

Johnathan today scared me to death. I fed him lunch around 1:00 and I thought to make him "Suddenly Salad" the ranch & bacon kind. I thought it wouldn't be a problem. But about 20-30 minutes later, he starts rubbing his eyes a lot (with food goober'd hands of course) and crying more. So I stop and get him down and after washing him off, notice his eyes are swelling shut and he's got erythema and red blotches all over his face. Ahh! He also suddenly had sneezes and lots of snot coming out.
At first I just thought he had gotten the sauce mixture into his eyes, so I quickly took him to the bath and rinsed his face a lot. He was soooooo sad looking, I almost cried. It kept getting so swollen in his left eye, he almost couldn't see out of it. So I called my pediatricians office and spoke with the nurse and she goes, "Wo!" and sent us in right away to see a doctor. Ironically we were going in anyway at 2:30 for a flu shot for him.
So we got there at 2:00 and of course it was a "hurry up and wait" situation. We ended up not being called back until 2:35 anyway. Kind of disappointing. But before I left home, I had given him 2.5 ml of Benadryl as the nurse instructed, so all his symptoms went away except for the hugely swollen left eye. The benadryl helped a lot. The doctor checked him and said, "yes, the dianosis is allergy". So I wonder what it was in that box that made him do this. Crazy! No more suddenly salad for Johnathan for a couple years! Man that experience freaked me out!