Four guesses:
1. Battery on the HRM strap is getting low. Check/replace the battery in the strap.
2. If you weren't sweating hard, could be poor conductivity between your skin and the electrodes, exacerbated by poly jersey flapping in the wind and the sensor is picking up static. Some spit on the strap electrodes should solve that problem, and you can buy electrode gels that do the same thing. FWIW, this happens to me usually in late fall or early spring, when I'm not wearing a jacket and not particularly sweating -- like right about now.
3. Asymptomatic atrial fibrillation. If neither of the above does anything, see a cardiologist. You may have to wear a Holter monitor for 24 hours to see if it recurs.
4. HRM strap is wearing out. Replace it and see if the problem goes away.
I'd probably approach it in sequence #2, #1, #4, and #3. Spit is cheap, a battery isn't too much more, HRM is less hassle than the doctor, and I don't want to hear AFib.
