Inflammation of the pes anserine bursa
The pes anserine bursa is a sac (lined space) under the attachments of the pes anserine tendons on the inner aspect of the knee. If it becomes inflamed (bursitis) it can cause pain on the medial side of the knee, with swelling and sometimes snapping.
Pes Anserine Bursitis. Mohseni M and Graham C. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Mar 21.
Painful Pes Anserine Bursitis Following Total Knee Replacement Surgery. Ariff MAM, Ros MIAC and Yahaya NHM. Sultan Qaboos Univ Med J. 2018 Feb; 18(1): e97–e99.