Could you send this picture to your vet? Explain you have an appointment on the 16th but by then the appearance may have changed significantly. It could be a hint of what’s going on or as you thought it may be just a hematoma. Is that the area where the drainage was done?It’s fantastic that...
I wonder if you could pack it with a long strip sterile gauze ( maybe 1/8 “ thick) and leave a “ tail “ out. It would need to be removed and replaced daily. Would allow to heal from inside out. Just a thought.
If the vet drains it again possibly he could put a small rubber drain to keep the wound open.It may require one stitch to hold it in place but maybe not if the wound is deep.
I’m fairly new to chicken so I’m so sorry I can be of no help. I just wanted you to know I’ve been reading your thread and I’m pulling for you. I do know the pain in dealing with a chicken you love who is critically ill.I’ll be thinking of you and hoping for a good outcome.