I agree with the advice your vet gave you. I completely understand how frustrating and scary it can be when your pet has something wrong with it and you don't know how it happened or what is causing it. Because of the acute (sudden) nature of the bump and the fact that she's still eating normally, it does sound to me like she's either been bitten by something or stung by something and is having an allergic reaction to it. I suggest you keep up with the Benedryl until your appointment on Monday.
Of course, it goes without saying that if your little girl should stop eating/drinking or develop other symptoms (vomiting, diarrhea, shortness of breath, lethargy) this weekend, then you should take her to the emergency clinic, however as long as she's acting normally for her, you don't need to rush in to see a vet.
I hope this puts your mind at ease a little!
I'm glad that your pup is feeling better and that it was just an abcess.
I've seen a few in the clinic I work in, but they didn't all come in around the same time and they most likely weren't caused by anything in common. Abcesses can be caused by many different things, the most common of which is a simple infection caused by staph that just gets out of control.
I don't think there is a way to prevent future breakouts...especially since we don't know exactly why your pup developed one. Hopefully this was an isolated incident and your little one won't have any further problems!
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