I think I know exactly what you are experiencing...

I partially tore my right ACL in high school and every so often the same thing happens to me that happened to you. It's usually if I'm sitting on the ground on top of my feet and I roll to one side to then stand up. It partially dislocates something. There may be a little bit of pain but it's more of a feeling that something slipped out of place. It usually eases itself back to normal in about a day or so.

I think it has something to do with the strength around the knee and the partially torn acl (it's just a sloppy connection and won't hold everything together as tight as a normal one). It is pretty common for people to slightly tear ligaments. You may have done this a while back and not have even known. Then the joint is a little weaker. Sometimes the ligament grows back on it's own and it is 100% again.

Unless it starts happening a lot, I wouldn't bother going to the doctor. There's nothing that they can do. Unless a ligament is fully torn, they won't operate.

I ended up completely tearing my left acl two years ago and got that surgically repaired. I'm just waiting for the right to completely tear in the next ten years so I can get that fixed.