If she has been stung or bitten you could give her some Benadryl and it should help with the swelling.If the swelling does not improve or if it gets worse then she needs to be seen by your vet like Ginger said above.Sometimes if they have been stung or bittten which seems to be what it is then it can cause alot of other problems.Benadryl liquid is quickly absorbed into there system and can help with stings,bug bites and allergies.I hope that she is feeling better soon.
She could have a bee sting, or some sting or bite. The safest option would be to get her to a vet, because it's hard for me to tell you what this is, as she needs an examination. On the other hand it could be an allergic reaction of some kind. I think the best thing is take her to be checked out.