Well Shawna, I know you feel helpless...poor babies~you & Sherman!
Yes, if they are allergic to fleas, ONE bite could do this to them, I too, think
it could be the problem as you mentioned finding a couple of fleas but that
was all. Or, possibly, on your waks, someone had newly sprayed their yard
with pesticides and he reacted to it.
Praying for the answer to be found, the vet having a solution and some QUICK
relief for dear Sherman & sending comfort to you, dear one. Hang on & GOD bless!