During sex heart rate goes very high as I have sick sinus syndrome?

Heart rate. Heart rate during sex can go quite high for some folks, see your doctor to be checked, may not be a problem, but could be.
Could be. It's possible. For instance, you could be getting paroxysmal atrial fibrillation during exertion. This can be easily determined by wearing a 24 hour holter to record your heart rhythm. (to be diagnostic, you know what you have to do while you're wearing it).