Not only do you have the right to make decisions about your own body, you have the right to practice or stop practicing whatever religion aligns or doesn’t align with your core values and beliefs. This is a wonderful time to not only explore self-expression, but to explore your value system and to think about how best to support that in your spiritual practices going forward. If the religion you grew up with doesn’t align with your values, it’s ok and very healthy to cut it out of your life.
As for your mother and your relationship, it’s ok that she’s upset. Adults get upset – it’s part of life. She will survive. Her emotional balance is not your responsibility. Her own behavior and attitude is what prompted you to not share your ear-piercing decision with her. If she wants you to open up to her more, she can make the choice to stop being so rigid and judgmental in her attitude and behavior with/around you.