I am now having the exact same problem for the last several months. I have seen a Dermatologist 3 times over the course of 6 weeks. He had me try two different steroidal cremes which helped reduce the size of the bumps but was not a cure. I've had this tattoo for 19 years until this rash suddenly developed. He said that it is likely the cadmium in the red ink that typically is the cause of sudden rashes. The good news is that he said it is not and will not become a tumor or cancer--the shin is just inflamed in response to the pigment. The doctor basically gave me the options of laser treatment to remove the tattoo or to cut the tattoo out which will leave a big scar. But I know this Dermatologist to be very conventional and mainstream his thinking so I am researching other options. Since someone mentioned a possible fungal infection as the cause of the rash I'm going to try colloidal silver spray for a few weeks starting tonight.
Hello,
Since the itching has started from the tattoo and then spread, so it is likely due to allergic reaction to the inks of the tattoo only. I suggest you to get an evaluation done from a dermatologist and till that time, take antihistaminics like Benadryl or Claritin and apply some topical steroid creams on the rash.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.