Hi. We need to first find out that does your knee cap pop out or your knee pops out. As both are separate entities and both have separate treatment. Knee cap popping out suggests that you may have a mpfl tear and knee popping out in other words instability while turning suggests acl tear. Clinical examination can give better answers to this and depending on that if required we might have to get other tests to find out the real cause.

Please get x ray of your knee. If there is no fracture then get an MRI Scan to rule out any ligament injury.