Forerunner 45 heart rate problem

Hi,

I have a Forerunner for 2 weeks.

The first week, the heart rate seemed correct varying between 60-100 and higher when excercising.
But a few days ago, the heart rate started to peak from morning to evening (up and down between 60-170), even if I am not excercising.

I checked how to wear the watch correctly and the cleanliness of the sensor.

What could be the problem?

Thanks in advance,

Best Regards

Lisa