Antihistamines decreasing HRV?

Hi everyone,

I'm dealing with long COVID for a few years and my doctor was thinking I have have a histamine issue so she recommended I start on rupatadine. I've been taking it for 3 days have since starting it (taking it in the morning) my fitness watch has picked up on my the being more elevated (from 59 to 64) and my HRV has decreased by about 15 points (it measures HRV through the night). The last two nights I've been having more vivid intense dreams as well.

Usually a decreased HRV indicates more sympathetic nervous system activity, which I already have a bit of an issue with. I am wondering if anyone else has experienced this and if it's a sign that this drug may not be a good fit.