Average heartbeat changes from february

SOLVED! Battery in HRM strap was dead... Replaced and will test in next run!

Until and including January 2020 my average heart rate was around 100. Starting from February, it changed to around 150. Average pace has not changed. I do not feel any changes when running, not short of breath etc. What could have caused this? A new firmware? I always upgrade to latest version as soon as it is public.