Knee pain with floating object

Patient: I’m a 24 year old female. I am a long distance runner, but I have taken the past 4 months off due to pain in my knee. I do have an appointment this week, but I thought I’d get a head start and wanted to know specific questions I should ask my doctor when I see him.3 months ago, I was sitting on a stool. I hopped down gently and I experienced the worse pain I’ve ever felt in my knee. It sort of locked up and I was unable to bend it without excruciating pain. That lasted a day or so, and the pain got better. However, starting that day, and every day since, there has been a strange feeling in my knee. It feels unstable–especially when going down the stairs. I also cannot squat comfortably. There is a hard floating object on the outside of my knee. At first, I thought it was a tendon or muscle because I could “pop” it back (I likened it to the top of a soda fountain drink–where you can push in “cola, diet, other, etc.”). When I extended my knee, it would pop back out. I should be clear that it is not my patella that is moving. It doesn’t cause any pain day-to-day, but I cannot comfortably run because it feels unstable.

Symptoms: Floating/movable hard mass on outside of knee, knee instability, no day-to-day pain, uncomfortable while walking down stairs and squatting