My aunt's swan had something that looked like that and it turned out to be a benign tumor called an adenoma but that kind of thing doesn't just appear overnight. Is it possible that your duck ate a bee/wasp and was stung internally? Have you checked under her feathers for an external wound? It's rare for birds to get stung on their bodies due to their feathers but it can and does happen on occasion around the head and neck area.
If your duck is eating and drinking without issue, doesn't appear to be in any pain and is acting normal, give it a day or two before you start to worry. If the bump shrinks then it's healing. If it grows or remains the same, have a vet look at her.