My Heart slows down and I'm worried

Ok, so whenever I will either be laying down or sitting down or pretty much not doing anything physical, my heart is fine...however, if i get up to do something, or especially if i get something to drink, my heart will slow down to a crawl and beat really hard for about 3 seconds and i have to take a deep breath to get it back on track.  not too long ago i stood up and took a drink of some pop and it seemed like my heart actually stopped for a second and then it beat really hard and i almost blacked out.  i took some deep breaths and then it got back on track and stopped beating hard...i am afraid.  it seems nobody knows what is going on...does this happen to anyone else...i dont know what it is.