Knee problem

[Makemegood]

My knees feel "loose" and i was wondering if this is a problem i can solve by merely training (any exercises in specific i should do?) or if i should talk to a doctor.

I train karate and when then knee gets pressured slightly in wrong direction it twists in and out of the joint. Happen if i just lift my leg to perform a kick or just standing up after sitting on the floor. Doesnt happen often and i can continue the training without problems but its rather painful, and im sure its not good for the knees longterm.