Possible allergic reaction to a spider bite

Question
My dog awoke on Sunday with a swollen face, a rash and puffy
legs/paws so we rushed her to the vet who diagnosed a severe
allergic reaction and gave her antihistamines, antibiotics
and a steroid injection. 2 days later and there hasn't been
much of a change. Her swollen face has gone down but seems
to have moved to her ears. She's drinking more (not
excessively) and only picks at her food. We live in the UK
and we noticed an unusually high concentration of spiders at
a local park we took her to on Saturday afternoon. Could she
have been bitten and if so what's the best treatment?  

Answer
Swelling of this sort will kind of move around from area to area before it resolves completely. You should be giving her Benadryl or some equivalent to that daily for the swelling.

Most of the time it is not a spider bite but some sort of a bee or wasp sting that does this. Certain weeds and plants can also cause allergic reactions.
About 98% of the time, however, it is a bee/wasp/hornet sting that starts this.

The antihistamines and the steroids will cause the extra water consumption. That will taper off after about a week or so. If you give her more antihistamines and she isn't better in 48 hours you need to call your vet back.