I have had this checked out before (about 2 years ago), the docs (4 of them) said it was anxiety, after I'd had an ECG and 24-hour ambulatory monitor. This particular time, which happened during masturbation, I'd been nervous because of some slight muscle soreness around my left shoulder/upper back/underarm/pec. area that has persisted for a few days and I'd been checking my pulse occasionally. I walked about a kilometre on my way home, fairly briskly, felt what was probably the muscle soreness and checked my pulse again, it seemed rapid but ok - consistent with physical exertion though perhaps I'm unfit also. About an hour after I got home I masturbated and noticed a fairly rapid heartbeat as I went at it more vigorously. After I finished, I took some deep, slow breaths and it was back to fairly normal after about 30 seconds.

It's also the first time it's happened in masturbation for a long time, possibly more than a year.

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