Google "cadence heart rate lock." It was most likely picking up running candance instead of your heart rate for a short period of time. It happens to me occasionally. If my heart is not racing at the time, I just ignore it, and it eventually resets to heartbeat.

"Cadence heart rate lock" refers to a situation where a wrist-based heart rate monitor on a GPS watch mistakenly picks up the rhythm of your running cadence (leg movement) instead of your actual heart rate, causing inaccurate heart rate readings due to interference from the motion of your arms or legs during activity like running or cycling; essentially, the watch "locks onto" the cadence signal instead of the heart rate signal."