You didn't ride near large electrical lines or another person wearing a HR monitor. Mine always goes haywire near electircal pylons and anyone elses. Even though mine is coded the signals often cause issues.

I too have a 34D bust and tuck mine a bit under my bra band (and they are underwire too). I wonder if you have enough salt in your sweat to conduct properly....

There is one brand of HR monitor that takes it from your fingers, however you have to hold two buttons- that's OK for the gym or runners, but not so good for the bike.