You may wonder how I figured out Benadryl might be an issue for me.
I went for a walk yesterday. I track these things on my Apple Watch which shows you your heart rate. I noticed it was at least 10bpm lower than it had been at a similar point in the last walk I did a week ago.
I realized I recently stopped taking Benadryl and wondered if it might have something to do with it. Turns out: it explains that AND my recent insomnia:
https://www.drugs.com/sfx/diphenhydramine-side-effects.html