Sometimes I can feel my heart stop and start to be really slowly, before it starts up again. What could this be? I haven t seen a doctor about this before. Once, when standing up, my heart started beating really slowly, and I couldn t see or hear anything for a 5-10seconds (this happened about 2-3 years ago). About 5 minutes ago, I noticed my heart racing (for about 10 seconds), then slowing down to a normal beat (again, for about 10 seconds), then start racing again.. until it went back to normal. I am 21yo, female, not overweight in the slightest. However, I do smoke (only about 5 a day, for the past year).