I hit my knee really hard when i fell skating. now everytime i stretch my knee feels dislodged, but its not swollen or bruised.

Probably bruised: Sounds like a bad bruise, but if you press down on your knee cap and your thigh muscle goes crazy or it hurts really bad then time for an x-ray. Rest, ice, compress with an ace bandage. If no better in 3 days or if stiffness is worsening time to go to your favorite orthopedist.

Damage inside the kn: It is possible to do damage to the ligaments or cartilage inside your knee. If your knee is locking or giving way, it may be necessary to get an mri.