raised bumps all over 3 tatoos

Have 3 tattoos, 2 on left shoulder blade (4 years old) and one on upper right arm (10 years old). A couple of months ago the 2 on shoulder started to get bumpy, not pimples, and mostly in the black ink. They do not extend into the surrounding skin. The 2 on the shoulder are almost copmpletely covered, while the one my right arm has only 3 or 4 bumps. What might be the problem. Never had this before. Thanks.