Small (1cm ish) squishy lump back of right ankle bone. doesn't hurt. feels kinda like a squishy vain. the lump disappears when foot is flexed. i'm 16?

Joint Swelling: First rule out joint swelling . Things are so tight down there the swelling from the joint sticks out between the ankle bones in back and in front. Flexing adds room to the joint so it seems to go away. Please get an x-ray to learn what is wrong. Most often, at your age, a piece of bone on the bottom of the tibia may be separating out called osteochondritis dissecans and needs urgent care.

Cyst: Get an x-ray to rule out a bone/joint issue. There may be a ganglion cyst from a tendon or the joint. See you local podiatrist or orthopedist