No, that's not normal. And you should go see a doctor. But sex probably isn't the root cause of your problem. 

In the most likely scenario, the physical stress of sex may be revealing some underlying health condition. From a medical perspective, passing out happens when the blood flow to the brain is restricted — and you need that blood to be flowing during sex.

When you're exerting yourself in the sack and your heart is racing, it's not just because you're excited. You're essentially putting your body through a cardio workout that's pushing your body's cardiovascular system beyond its resting state of comfort.

If you have some existing condition, like dehydration or stress, that exertion might simply be too much for your body to handle.

It could also be related to alcohol or drug use, or in rare cases, passing out might indicate a serious heart condition. You should take this problem very seriously. 

If your doctor doesn't find a medical cause, you might want to speak with a different sort of doctor: a psychologist.

Sometimes, in acute cases of post-traumatic stress disorder, a traumatic sexual memory can trigger physiological responses ranging from vaginismus to blackouts.

It could be that certain sexual situations prompt your subconscious to shut down.

I strongly encourage you to see a doctor — and soon.