GI surgery 10 days ago - how concerning is this??

He got his cone off today while his onesie was in the wash so that’s where the fresh blood is from. But inspecting it I noticed this huge lump at one side of the incision. It’s been 10 days since the surgery, how concerning is this?? How long do I monitor it before taking him back in to the vet? The second picture is from 3 days after the surgery, the rest are different angles of the site from today. I tried to palpate it and see what it feels like, I can’t tell if it’s “hard” or “soft” like what I’ve read online, it doesn’t feel soft like a water balloon but it also doesn’t feel like he has a rock under his skin so I don’t even know. I’m freaking out.

He’s a 1 yr old male cat, neutered, indoor-only, no relevant pre-existing conditions. The surgery was because he stopped eating after throwing up a shoelace but there ended up being nothing in his stomach. He’s been roughhousing with the kitten a lot more than I’d like but I’ve been trying to keep him sedated and keep him in the bathroom when I can’t supervise them.