Hi phoebewhite,i found the following ......
""Although such a feeling is usually completely normal, it may be unpleasant. For instance, people often describe hard beats, fast beats, irregular beats, skipped beats, fluttering, “flip-flopping” or a pounding sensation in the chest or neck.
Palpitations are defined as an unpleasant awareness of heart muscle contractions in the chest. Although palpitations are very common and usually completely harmless, they may occasionally be a manifestation of underlying heart disease.
Palpitations are a very common reason people visit their doctor. This is not surprising as symptoms associated with the beating of the heart are likely to get our attention and they often make us anxious.
However, it is important to keep in mind that palpitations are more often caused by the anxiety itself than an underlying heart disease.
The key thing is to understand that palpitations are a sensory symptom, an experience or a feeling. As such they may be difficult to grade or measure. The description often varies a lot between individuals.
While some people seem to be aware of their heartbeat at all times, others have almost never felt their heart beating. Obviously, the latter group is less likely to complain of palpitations.""
Also found this ....
Q. Why does laying on your left side cause you to have more palpitations and pounding heart?
A. Palpitations , pounding heartbeat and increased cardiac awareness is quite common when you lie down and, in particular, on your left side. This is due to a positional shift in blood flow between different parts of your body and, in particular, the heart, which makes it start doing this. It is not anything to be concerned about, but it is noticeable in some patients and simply changing positions usually stops it.""
I would recommend get an up to date evaluation by talking to your GP about your concerns or Cardiologist
who could provide Holter Monitor recording and ECG .
Good Luck and Good Health.