It could be an abnormal heart rhythm, including but not limited to premature ventricular contractions (PVCs). You should see your doctor for further evaluation with EKG, Holter monitor, event recording, etc.
Thyroid issues are associated with ectopy formation so yes, what the above user said. A holter is best.
Thyroid issues and reflux can cause palpitations, always safer to get checked out by a cardiologist to make sure they’re benign. That flip flop feeling is usually a PVC. They are generally benign unless you have structural heart disease or some congenital rhythm defect. But just to be sure get checked out.
Thyroids also associated with AFib and Afib triggers as well. Doesn’t sound like it’s the case here but is apparently a thing to keep in mind.