I have a tattoo that was done by an amateur on my upper left arm 9 years ago. sometimes, but not always, it burns and is tender to the touch.

Bad ink: It was likely done with a conveniently available "ink" that is not meant for human / skin contact. Your body is reacting to it, and it will likely get worse over time, eventually eroding to the surface. See a dermatologist or plastic surgeon who can help--likely by abrading down to the ink or lasering it to see if stabilizes. Good luck!

May be: You may be partly reacting to the dye in the tattoo.