I was on about 300mg of quetiapine when i started having a fast irregular heartbeat. But then I realised I had a cigarette, downed a can of monster and and cycled a bit. But I seriously thought I was going to have a heart attack if I carried on riding my bike. I had to stop and sit down and try the whole "deep breathing" stuff doctors are always trying to say. Then it actually slowed down a bit. My dose has since increased and I haven't had that again. Perhaps for you it was a combination of things, anxiety? excited about something? caffeine? idk just throwing out some ideas