Heart rate suddenly accelerated during mild exercise?

By way of background, I'm at a modest level of fitness - I often do a bit of running or casual exercise, though running any sort of distance is pretty intense, and it's quite ordinary for my heart rate to enter the range of 90-100% of maximum while running.

However, I pulled my calf a few weeks ago, so haven't really been exercising since, until this evening when I tried playing badminton...which *can* involve very heavy exertion, but this evening didn't. I was still trying to take it easy on my calf, which meant that I wasn't really doing a lot of the usual anaerobics associated with the sport. In fact, it was a pretty relaxed pace overall.

Then, after playing for a while, my heart rate, for no apparent reason, suddenly increased to an extremely high level - I would estimate that it reached or exceeded my 'maximum' heart rate. There was no pain, no difficulty breathing (I wasn't even winded), and the only real discomfort was the pounding sensation itself, which was more intense than I usually experience even during high-intensity cardio workouts. I took a break for a few minutes and it returned to normal.

I guess I'm wondering how worried I should be. I've never experienced anything like this before, and it's a bit unnerving. What might have caused it? Is it likely to recur? Could it be a symptom of a more serious condition?