It still would be a good idea to have her checked over by a vet.
Vomiting large amounts of
mucus, unexplained neck swelling, loss of appetite, and lethargy are all symptoms which need proper attention.
Yes, sometimes dogs have an upset, then get better of their own accord, and we are glad we didn't panic (and run up a bill at the vet's office!) Sometimes that happens. But it is a fine line to balance. In my opinion, anything that is pretty dramatic, of sudden occurrence, or worsening.