Probably the most uncommon stung is in the mouth when a bee is swallowed unwillingly [while eating bread and honey or whatever] This of course maybe life threatening.
I personally never saw such a case or even heard about this.
What can you do before getting medical help?
Well if you have a antihistaminic or prednisone at hand, take this straight away before you get emergency medical help.
But these may work too slow!!

A few people may always have a EPIPEN with them.
The Internet is remarkable “useless” here concerning emergency advice. [try to search]

The best advice I saw is from Dr. Alfred Vogelz”l -a famous naturopathic doctor,known best from his echinaforce.
His advice: gargle repeatedly with salt water – two tablespoons of salt to 100 ml of water. This may save you before you get real medical help.The poison maybe partially “extracted” by the salt water.