Stray cat has a mass under her nipple

We took in this stray cat. She was pregnant and then came back a couple days later not pregnant. We figured the babies were dead as she showed no urgency to get back to them and was not lactating. Her nipples were also clean.

We took her to the vet for shots and tests and then took her inside. A couple days later I find this lump on her lower abdomen. Any idea what it is? It developed so rapidly cancer seems unlikely, but there's no redness or heat for mastitis.

UPDATE: Could it just be milk build up?