Is it possible what you're finding as a lump could be the little ducks craw? This is at the front of the neck above the breast bone and can become as large a a walnut in this age duckling. The pinkness you're seeing may also be the skin with the down thinning due to the stretch. The craw is where food is stored until it's digested. It's a very good idea to provide something in the form of gravel, sand, this helps grind the food in the craw. For very young chicks and ducks I used parakeet gravel. A few good pinches when ever I freshen the food.
This might not be on target but a full craw on a small duck can very well look like a swelling. The swelling should be less promient the first thing before morning feeding. And will increase in size as the day goes on..
Hope will Beacon grows to be a big guy/girl. So cute.