I was doing plank every day during lockdown a few years ago, and it's pretty tough. Don't think I can do it now again, because like you my arm has started to hurt recently. I'm surprised I managed 100 squats on the first day, even though it nearly killed my legs afterwards!!
Good luck with your guess!
Yeah, I can imagine the feeling, especially the next morning when you start to feel the pain the muscles... 🤐