Haven't yet determined what was the source. The only things I had eaten recently were cereal that has always been safe and a glass of orange juice. About half an hour after eating, my tongue and lips started getting tingly and I noticed a few scattered hives on my chest and stomach. As per our plan, I dosed with Benadryl and continued to monitor. I never had any trouble breathing, no vomiting or GI troubles, and the tingling went away shortly after taking Benadryl, although hives continued to crop up on my back and arms for the next few hours. The hives were gone by early afternoon, although I woke up this morning with more hives on my chest, back, and arms, which have been coming and going all day. No more recurrence of tingling lips/tongue.
Now I'm just trying to figure out the source...the most likely culprit is the cereal, although I've never reacted to it before.