Your day of gardening sounds like way too much for you right now - you should really pace yourself
I really feel for you with your back. I have had two back surgeries for slipped discs and currently have one bulging apparently but knock on wood
my back has not been bothering me at all. I would suspect posture first; are you still mostly on the walker? If so, you might be ready for the cane and therefore be more upright. Check where you are sitting to see if you have a sizeable hollow behind your back; if so tuck a little cushion or a folded towel to fill that space. Are you sleeping on your back still? If so, put a pillow under your knees and if sleeping on your side ask hubby to tuck a pillow firmly into your back so you are supported without your body making that effort. Try stretches by reaching for the sky one arm at a time. A great PT exercise I was given following back surgery for general aching was to put my finger tips on a window sill for balance and raise on tip toes, hold to the count of 3-5 and lower. Repeat 5 times. Keep good posture when doing that. Not sure is any of this will help you but I sure hope something does. Back pain is exhausting!