Hi, thanks for writing to HCM.
The fact that the swelling is disappearing while lying down and presenting in standing position indicates that the swelling contains fluid (possibly blood) and is arising from the joint. It could be a HEMATOMA
inside the joint.
Presence of a post operative hematoma in the joint is not a good sign as it may get infected and causes havoc.
So I suggest you to consult your treating doctor and rule out any serious issues.
Hope this information is helpful. Good day