Hi, haven't done any posting here but I've got a bit of personal experience with this one so thought it might be worth chiming in.

It might be a fluky reading, but there's also condition out there called SVT that sounds a bit like what you describe. It doesn't happen to me much but sometimes when I'm out riding sort of hard my heart rate will shoot up quickly to 230-240. My max "normal" heart rate is ~195. If not for the heart rate monitor I wouldn't notice anything weird was happening. Everything feels completely normal. In my case it doesn't happen frequently and my heart rate comes back to normal after a minute or two if I stop or ease up so my doctor concluded that it's not really a big deal, but the moral of the story is don't just say "that's impossible" and ignore it. Your heart can beat faster then 220 - your age if your body tells it to. If it happens again take your pulse and if your heart is really beating that fast error on the side of caution and talk to a doctor.