my 4yr old boxer has developed what seems to be fluid accumulation on the side and top of her head. she is behaving normally, but the swelling reaches from behind her eye to behind and below her ear. my local vet says that it is not a spider or snake bite b/c we would see the tissue degenerating. it first appeared 6 days ago and has not subsided, despite being treated with benedryl, as was reccommended. my concern is that it may be something worse. also it is not sensitive to the touch and there is no brusing, making me doubt that it is from some sort of trauma. in addition it appears that the opposite side is sunken in, and not just because the otherside is swollen. there are significant changes in the contour of her head, almost like there is a lack of fluid or tissue on the side opposite the swelling. the vet checked the lymphnodes and found no problems. any idea what's wrong with my baby?

Optional Information:
Age: 4; Female; Breed: boxer

Already Tried:
local vet