I know someone who suffered from the same problem. It apparently is a rare sleeping disorder. Go to a sleep clinic ! It is very harmless apparently but could be dangerous if untreated (at an older age so don't panic) ... I will ask him what the disorder is called and update my answer, so check back !
PS. I am pretty sure it is the same condition as he explained the symptoms to me in detail before the diagnosis was made. For years his condition was not diagnosed properly until he went to a sleep clinic.
UPDATE: I just called him. The disorder is called "Sleep Apnea"
You can find more information here:
I think the feeling you experience during the day however are anxiety
attacks because you are worried about your nocturnal experiences. See a doc and go to a sleep clinic !
I'm not a doc but can tell you it's not your heart so ...
DON'T PANIC ! You'll be fine !