Why sometimes I feel dizzy when I laugh strong?

Need check up. Sometimes laughing hard may momentarily drop your blood pressure and/or lower your heart rate.When this response is exaggerated it might make you feel lightheaded. I would see a doctor and undergo an examination to make sure there is not a condition known as hypertrophic subaortic stenosis. Also perhaps recording an ECG strip as you laugh may help to see if you have a long pause in heart beat.