My mom used to be the same. The worst moment for me was after I had mostly completed my sleeve. I had my arm up behind my head watching TV, and looked over, catching my mom staring at my arm with a disgusted look. I asked "what?" probably too defensively, and she told me I had ruined my arm. Things were a bit tense for a while.
For us, the answer was for me to explain how hurtful those kind of reactions were (and I totally got the "it hurts me when you get more tattoos" response) and try to show how important they were to me. I think my mom genuinely saw them as a phase that I would grow out of and regret. Once she realized that wasn't the case, and I continue to get more coverage, she's accepted it and usually comments positively when she sees my new tattoos.
I guess I recommend trying to sit her down, and convey how much both tattoos mean to you and how hurtful her actions are.