My arms no longer hurt. And my back doesn't hurt either because I'm making modifications to protect it.
Deep in my heart, I've finally come to accept my practice as it is. At the moment, it's not that great. And that's fine.
I'm backing off, listening to my body (something I always implore my students to do). I've started to modify without feeling like it's cheating. My strength is coming back quickly - Chaturangas were better today. Soon I'll be doing vinyasa between sides again. The biggest issue is now flexibility, specifically, my hamstrings. And the tight hamstrings are contributing to the cramping in my low back, I'm certain of it.
My hamstrings are really, really tight. How tight? Well, I'm barely resting the tips of my fingers on the floor in Uttanasana and using a yoga brick for the standing postures. My head no longer reaches the floor in the Prasaritas. For the time being, I've swapped out Janu Sirsasana with Supta Padangustasana, but using a strap. I'm doing everything I can to gradually lengthen my hamstrings.
Funny, I can do Utthita Hasta Padangusthasana, I just can't straighten my leg :-D