Thanks for the answers to my questions.
I'm assuming when you said "watery" lump, you meant that it was soft and not weeping/oozing, correct? If that's the case, then I have several thoughts as to what this might be:
1. Hematoma or blood filled space or
2. Seroma which means serum filled space.
Either one of these conditions can develop secondary to trauma where damage is done to cells and they essentially leak into surrounding tissue. What sort of trauma occurred may not ever be known, though.
The body will eventually absorb the fluid inside of them although it may take several days to do so. I usually dispense antibiotics for these patients since bacteria may grow and an abscess can develop. This isn't actually a very common scenario but it can happen.
3. An abscess although I would expect that you'd see evidence of a wound of some sort on the surface of her skin. Also, it's not painful which abscesses tend to be.
If you were concerned about cancer, I can absolutely reassure you that this isn't likely to be her problem. Cancerous growths can develop quickly but not to this size in such a short period of time.
If you wanted to apply a warm compress to the area, it won't cause harm but as to how helpful it will be, remains to be seen.
This wouldn't be an ER type of situation; however, if the swelling isn't reducing in size, a vet visit might be prudent to confirm that this is a fluid filled space (your vet would aspirate the lump).
I hope this helps to reassure you about her. Deb