Welcome to Helath Care Magic.
All lumps are not cancerous.
Based on the information provided by you , the lump could be synovial cyst
of knee or bursitis
or subcutaneous swellings or lipoma
or vascular swellings.
You haven't mentioned whether your mother had any pain or whether the lump is hard or soft to touch.
You have not mentioned whether your mother had been suffering with Osteoarthritis
(OA)knee. In Case if she had OA knee then synovial cyst or bursitis are the most probable possibilities. In Case if she has no OA and has no pain while touching the lump then the likely possibility is Lipoma or sub cutaneous nodules. Do not worry. They are highly unlikely to be cancerous. So, you can rest assured and can wait for another week.
Get and ultrasound
of swelling done to rule out all your doubts.
Wishing your Mother a speedy recovery.