If you lived near I would think it's the groomer I took Georgie too! The same thing happened to Georgie!!!! It was before he was neutered and we took him and when we brought him home he rolled over on his belly and immediately noticed it
I called the groomer and they accepted blame but didn't act like it was a big deal. I was p*ssed like you are! I am very sorry this happened to you too! Hope your baby feels better soon.