It could be an allergy or contact dermatitis that flares up when she comes in contact with something she is sensitive to. You can give her 1 pink Benadryl pill (or generic/store brand) 25 mg. and see if the swelling goes away or starts to abate within a half hour. I would also try to get her to the vet's when she is in one of these situations so he/she can see first hand what it looks like.