I haven't been diagnosed with FM, but I do have palpatations. Feels like a "flutter" or a muscle twitch for me. It's very scary. I had an atrial fibrillation episode back in 2000 and was in the hospital for 2 days, so every time I get a flutter, I freak out.
I also have panic disorder and anxiety. I think that palpatations are a symptoms of these as well.
I've done all the tests too and they didn't find anything. I had an ER doc tell me once that I had an enlarged heart, but I haven't had anyone else confirm it.
Best wishes to you