At 50 kg and age 21, I would doubt that your problem is sleep apnea. This really sounds like a minor problem. I would not worry about it at all. A few skips and bumps every so often is likely nothing at all to worry about especially since you had all the tests and everything shows nothing. My daughter at your age had the same thing and the cardiologist did not even do tests. He said it is normal. If the doctor said you are fine, relax and ignore occasional bumps.