I don't get site reactions from Humira, but they are fairly common. Your skin is reacting to something in the injection, whether it's the medication itself, the carrier fluid, the needle cover, the alcohol used to clean the injection site, etc.
You might try icing the injection site before and after the injection to see if that helps reduce the reaction. You can also try using a cortisone cream on it.
That sounds like a fairly large area, so it's good that you have reached out to your doctor. She will let you know whether to be concerned or not.
I have a friend with psoriatic arthritis who has been on Enbrel for years and gets site reactions similar to yours.
Humira is a fully human monoclonal antibody, so no mouse protein.
Ginny - 57 year old female
Dx with USpA in March 2013; changed to AS in July 2015
Iritis and Scleritis
unicompartmental knee replacement June 2014
MTX, Humira, Cyclobenzaprine, plus Indomethacin ER as needed
Supplements: Folic Acid, Vitamin A, Vitamin D, Calcium, Fish Oil, Melatonin, Culturelle probiotic