Why does my tattoo do this?

I have had it over a year and every now and then it starts to itch HORRIBLY! If I so much as tap it the itching only gets worse. Sometimes it gets swollen too. I’m pretty sure the swelling is just the outline of it. I have gone to a few different artists to get their opinions and they all said it was probably still healing… That made sense but it still does it after a year!! When I asked if I could be alergic they said it is very unlikely, but more common with red ink. (My tattoo has red ink.) It’s not the red part that itches though! Could it STILL be healing? (It’s a pretty big tattoo…)I have another small tattoo that is all black and mostly outline, and I’ve never had problems with it.