If the bump is rock hard, it's most likely a tumor. Not a bite which would feel soft and water-filled, much like a cyst. A bite often has a tiny scab in the center where the skin was broken during the biting from the bug. If you can feel a scab in the center, then it's a bite and will go away on its own.
Not all tumors are going to kill a chicken. I have a thirteen-year old hen with a tumor in exactly the same place as your chick. She's had it for around six years, and though it slowly grows larger, it hasn't killed her. She's the oldest chicken in my flock.
All you can do it wait and see if it gets larger and if it starts to make the chick sick. If the chick gets sick, then the tumor is much larger below the skin and maybe spreading.
If the chick doesn't behave sick, it may not be a serious tumor.
Not all tumors are going to kill a chicken. I have a thirteen-year old hen with a tumor in exactly the same place as your chick. She's had it for around six years, and though it slowly grows larger, it hasn't killed her. She's the oldest chicken in my flock.
All you can do it wait and see if it gets larger and if it starts to make the chick sick. If the chick gets sick, then the tumor is much larger below the skin and maybe spreading.
If the chick doesn't behave sick, it may not be a serious tumor.