If it keeps expanding quickly it could be a cyst it will need to be drained and treated
Hopefully this might get picked up by someone who might know more about it but my hens lump was nearly touching the ground it expanded that fast she's a lot better since I drained it
if its soft and just under the skin it very similar to her lump when I first noticed it
I made the mistake of leaving it and it hardened up in places so I would recommend not to leave it to long as it will be difficult to get out the pus