Red dots on an OLD tattoo

My girlfriend's tattoo is over two years old now but she's only just starting to get red bumps on the area of the tattoo. They've started spreading to around the area in the perfect shape of the tattoo (a moth). It's a recent occurrence though and in her old tattoo, not her new one. Everything I've seen about problems like this talk about it being a reaction to red ink or the lotion but 1: there's no red ink in her tattoo and 2: she uses the same lotion on her new tattoo and there's no reaction. It seems a little odd that after two years of having it she's only just now getting a reaction. Any suggestions as to what it could be?