I'm in my late 30s, riding 100 miles a week with no known health problems. About once a year for the past 25 years I get the palpitations described in the link below. Normally, I'm off the bike when these happen, so they go away after a few minutes.
But this time I was on the bike, riding to work at only medium intensity, when they started up. I pulled over for a few minutes and they seemed to slow down enough. But then when I started pedaling again I noticed that the heartbeat would get mildly uncomfortable once I started picking up speed. I was able to finish my ride, but only at low intensity to keep the heart rate down.
Time to see a doctor?