Hi all, first of all, I hope Darwin is doing better. I'm sorry that you had to go through additional surgery of the injection site. Caring for my kitty post-op was so challenging and heartbreaking. I hope his pain has decreased --- it's really hard to watch them in pain without knowing how you can help them.

I wanted to add that my cat, Phoebe (4-year-old rear leg amputee for injection-site sarcoma), also has this issue where she bucks her butt in the area and goes backwards. She's about two months post-op and definitely has a large nodule of scar tissue near the end of her incision site. When she got her staples out two-weeks post-op, the surgeon said the incision looked good and the scar tissue would eventually go away.

Her episodes usually last only a couple seconds and then are over, but she has an intense episode the other day where she was not even able to stand without bucking and moving backwards. We massaged the site and eventually, she hopped out of my roommate's lap and walked normally. 

I'm not sure that having her on pain meds every day is something we want to do, but I need to take her to the vet in about a month for her annual check up anyways.

So, you are not alone, and though I'm sad that Darwin had to go through this, it is comforting to know that other cat parents are facing similar challenges and are here to help each other.