If you are experiencing irregular heartbeats and constantly feel weak, it is very important that you contact your health care provider. You could be suffering from a disorder known as atrial fibrillation. A-Fib is a very common disorder of cardiac rhythm. It is easy to diagnose with an electrocardiogram (an ultrasound of the heart). There will be a few cases where A-Fib comes and goes, but most cases have persistent A-Fib. Fifty percent of those who are diagnosed with A-Fib also experience sleep apnea.