I think buttercups would give a localised rash round her mouth/muzzle. But runny nose and small lumps is defo an allergy. Could be anything. Have you changed her feed recently or moved fields or changed your hay supplier? It could be an allergy to insect bites, though I wouldn't have expected a runny nose. I'd say give her some tlc and if it doesn't clear up or gets worse or your ned shows other symptoms, a call to the vet would be safest x