Hi
A torn ACL has rather a different symptom set, focused predominantly on instability of the knee or feelings of the knee giving way. When the ACL tears, it often does so with a loud POP! followed by sudden and marked knee swelling. After that people often feel that they cannot trust the knee as it does not seem stable any more.
The sort of symptoms you are describing sound rather more like kneecap symptoms, which may often come on after an injury because of wasting of the quadriceps muscle, which normally helps the kneecap to align itself properly. Read up about simple quads strengthening exercises and see if that helps your symptoms.
KNEEguru