Possible it was a peck injury. I did notice some mild aggression at the same time cause by over crowding. I had no idea they would go so quickly. The aggression resolved as soon I increased their brooder. I'll definitely be more sympathetic to people that approach me with chicken questions...
I think my chick had a similar injury. I am a veterinarian with no poultry experience! My chick obviously had an abscess. I lanced and drained the abscess and soak in Epsom salts and bandaged with triple abx ointment daily. Also, treated with penicillin with no response so switched to...