Reactions to hornet, wasp, or bee stings

This is more of a public service announcement than anything else. Exactly what constitutes an allergy from a normal reaction to a bee or hornet sting?

After my second sting of the season - it's only mid-June - and with a swollen lower leg, I was curious.

I found this online and it was helpful, from North Carolina State University. Swelling and itchiness the day after is normal.

http://www.cals.ncsu.edu/entomology/apiculture/pdfs/1.09%20copy.pdf