Maybe you got old and your HR isn't what you think it should be. <grin>
I have heard a few others that claim when they put their garmin pod on other manufacturers straps that they got odd reading. Though typically I remember them being that they were reading much higher HR than what their actual was. I don't remember what brand strap was a problem for them though.
If this is a Garmin HR pod on a Garmin strap, then call Garmin. They've always gotten me straightened out. But call, don't email.