No your heart does not stop when you sneeze.
The changing pressure in your chest due to sneezing also changes your blood flow, which may change the rhythm of your heartbeat. The next beat may be more forceful and noticeable which leads to the ‘skipping a beat’ sensation.

  The origin of “Bless you” comes from a couple things.
When the Bubonic plague was spreading around, sneezing was a common symptom. People would say “god bless you” in hopes that the prayer would protect them. Some cultures believed that sneezing would release evil spirits which would then be exposed to others. The blessing was said to protect those that sneezed and those around them.