Don't 热敷.
I had exact the same when I was little, because I was too nervous when the doctor gave me a vaccine. It was on my arm for years and was bigger every time I did sports or felt hot in summer. I can not remember when it's gone, maybe at the time I was not afraid needles any more.
You need to relax, pay attention to your baby's behavior. No worries if it does not bother him/her. Take your baby to doctor if you notice the following:
baby cries a lot and usual comfort strategies do not work;
baby always tries to reach that hard bump spot;
the arm is weaker than the other one;
the spot is getting bigger and bigger (of course when you don't 热敷)
any other abnormal behaviors, consistent fever ........
