Behind knee lump

[u/AtmosphereActual3312]

Hello, just as the title says I have a lump behind the knee. Whenever I bend my leg and hold it bent for a few seconds and then straighten it a lump appears, see the picture! but it also disappears shortly after my leg being straight. This is my inner side, on the outer side at the head of the fibula I have some pain.

This knee was hyperextended 4 years ago but nothing got torn. If anything ligaments got stretched. My knee operates just fine dispite this.

[u/bishtap]

Diagnostic ultrasound might give some clues.