Is it poison ivy?
If not just wait a week and see if it subsides and then inject again,
Might develop a tolerance, I doubt it is related to Test E,
May have been the injection, or something else weird.
I have done dozens of cycles, and sometimes, like currently I think the gear is messing with my system, or I get a rash or something, usually just have to wait it out.
Unless you gave him T-400 or T-300,
That is going to hurt always.