Three year old dashund has a growth on the side of her neck. It appeared on Saturday, no sign of a bite.

It is possible it is just a fatty mass (puss filled). They can appear rather quickly - although I doubt overnight. Is it possible you just noticed it for the first time? The only place to get a firm diagnosis will of course be the vet. I personally don't see the necessity for an emergency visit - but I would make an appt.