Charge 4 is tracking my heart rate very inaccurate

[LucyWF]

For the last two days the heart rate monitor on my Charge 4 has gone wild! Yesterday it recorded me as doing over 700 'zone' minutes, with my heart rate being recorded as well over 100 bpm even lying in bed or sitting at my desk. So far today it has recorded me as doing over 300 'zone' minutes.

My usual resting heart rate is somewhere between 55 and 57 bpm. I am not feeling ill and I'm still alive, so there's no way my heart rate is actually 136 bpm right now, sitting at my desk.

Unfortunately, it's now useless for recording any actual sporting activity, or for getting any sleep data (which it says it cannot measure accurately).

Any ideas? I've not changed the strap. I've reset the fitbit, and turned the heart rate monitor on and off. Fitbit's chat function employee suggested that it might be because it's hot here right now, and temperature can affect the accuracy of its monitoring (?!) I had a short swim the other day, but the Charge 4 is waterproof, which is why I got it, and I measured the swim as exercise.

Any thoughts welcome!

Moderator Edit: Clarified subject

[MarreFitbit]

Hi there, @LucyWF. Welcome to the Fitbit Community Forums. Thanks for the detailed information about the issue with your Charge 4 and your time in trying to troubleshoot it before posting here. 

I've seen you contacted our Support Team after posting here and they've already assisted you. If you still need help or have any questions/concerns regarding the outcome of your case, please feel free to contact our team back for further assistance. I'm sure they will continue assisting you based on what you guys have already tried.