Sometimes milk ducts get blocked. Have you tried a warm moist compress to see if it helps with the pain? I have had similar issues (pain, no lumps, no obvious cause) in the past and a warm compress did ease the discomfort. It took a few days to resolve.
Next one, see if you can palate a location. I'm curious, and I get fascial pains (akin to fibro) between duct and fat tissues on occasion. It is something to ask about at your next annual. (PP has been my rock for that kind of stuff.)