Thanks everyone for the comforting replies. Knowing that it is a common thing makes me feel much better. One doctor told me that it probably has to do with playing the violin and lifting weights. That sounds reasonable.
I've heard of this before and looked it up again because I couldn't remember the name. The fancy word for this is fasciculations, in other words twitching. Our muscles can do this at any time. Remember the times your eyelid went twitching? Annoying as heck. People who work out may feel this twitching now and then, usually in the legs or chest depending on their workout. Other causes can be stress/anxiety, dehydration or low levels of calcium or magnesium. But for the most part, muscle twitches tend to be normal unless you have other symptoms to go with them.
I would say if this is happening nightly then ask for a holter to get it captured so you know exactly what you are dealing with. If not every night then a 30 day event recorder. If you haven't already let your cardiologist know what you are feeling and ask that you get this diagnosed. With your heart being healthy you may not need worry about what it is but I think it prudent to investigate it a bit more. Take care and keep us posted on how you are.
I am interested to hear of others with this sensation also. I used to panic thinking it was my heartbeat, but it has nothing to do with the pulse. My husband, also an AFib sufferer on some of the same meds as me gets this too, so I am inclined to think it must be something to do with medication. We describe it as a deep vibration, but it sounds like the same thing. What meds are you on? Seems also like it is nothing to worry about!