It is possible to dislocate a joint and pop it back in, if you have EDS Hypermobility type. I have it and yes I've popped a knee back in when I tumbled down the stairs. You are not absolutely fine after, the joint remains unstable and inflamed for a good while after, not to mention other injuries as a result of the fall, mine being spinal damage which is going to need surgery 6 years after the event. Hypermobility can run in the family though so there's a chance it could have happened, but you don't just bounce back to being fine straightaway.