Wecome to HCM,
After going through your history ,you had some kind of rythum disturbance during pregnancy
that is why you worried.
If there is no other symptom with slow heart rate
there is no problem, as you are feeing pain relate dto movement it is unlikely to be due to heart but it is better to check it with Physician and get 12 Lead Electrocardiogram and if every thing is fine then possibllity of muscular pan or back pain
is there then simple analgesic
with local hot foentation will help to give some comfort, but if it is severe enough to be concerned then a consultation with Orthopedic surgeon is what is required then a S ray of spine or MRI may be required.
Another possibility is hyperacidity
with reflux of acid in food pipe som etime produces pain which will be clear after examination by doctor.