Hi, thanks for writing to HCM.
Based on your description, a small lump below the arthroscopy
portal is definitely not normal but may not be a serious problem which is alarming. As you have no signs of infection in the joint, there is nothing much to worry about. Most propably, it could be either a seroma
or a local stitch abscess
Approach your treating doctor for further examination and evaluation.
Hope this information is helpful. Good day