These are Cysts (fluid filled sacks) that come from the Cartilages (Menicsi) inside your knee.

The Cartilage (Meniscus) tears.

The tear in the Cartilage (Meniscus) then acts as a one way valve inside your knee.

When you exercise, the pressure inside your knee increases, this forces fluid from inside your knee through the tear.

The one way valve stops the fluid coming back out.

Therefore the cyst gets bigger.

Occasionally the valve releases the fluid and the cyst goes away.

These are best seen on a MRI Scan (Magnetic Resonance Imaging Scan).

An operation to look inside your knee with an Arthroscope (Scope / Telescope) allows the tear to be seen and removed.

Once your tear is removed then so is the one way valve and the Cyst.