I'm an anaesthesiologist and we have equal training in such things in Australia.
The heart is normally predictably rhythmic - that doesn't mean it's always the same. When you breathe in, the rate should increase slightly; when you breathe out fully, the pace will soon slow a bit. You will notice this change more if your heartbeat is generally slow - that is, slow when you are fit, or slow when you are resting. If your heart rate is usually less than 60, this difference in normal breathing is detectable easily.
HOWEVER it is completely possible that you have an arrhythmia. We on the forum cannot figure that out from here, but the way you described it to me sounded like "vagal stimulation" was a possibility. Go to the doctor and explain very precisely when your heartbeat is irregular. One possibility is atrial fibrillation, which can be brought on or made worse by cold and by lying down! Repeat, I cannot guarantee this diagnosis. Slow atrial fibrillation is usually not a big deal, until it gets really really slow and you feel dizzy. One way to diagnose the problem is to have a simple electronic monitor taped to your torso for 24hrs. Check out this webpage:
http://www.afibbers.org/victor2.html
Here is a small sample from that page:
"A. Vagal stimulation
For patients with VMAF, vagal stimulation can bring on or terminate episodes of AF.
This includes activities such as:
? bending down, sitting down, or lying down
? drinking cold water or eating cold food (e.g., popsicles, frozen yogurt)
? jumping into a cold swimming pool
? gas buildup in the stomach
? eating a heavy meal
? coughing hard
? alcohol ingestion
? straining with a bowel movement.
These activities are especially likely to bring on AF in late afternoon until early morning,
when vagal tone is predominant. So can vagal maneuvers (carotid sinus massage,
particularly when lying down, Valsalva maneuver (sitting, bending forward, and trying to
blow out), plunging face into a basin of ice water while holding breath for half a minute). "