There are muscles below boobs and near boobs. As well as every “sensitive area” on both sexes bodies. It feels very different when someone is touching these areas with massage intentions vs sexual intentions. Assuming she’s had anyone touch her body at all (sexual or otherwise) she can surely tell the difference. You could (with her permission) give her a full on boob massage (massaging the pecs and the other tissues around the breasts) and it still wouldn’t be sexual unless you started touching sexually. Ie. Nipple touching, caressing the skin and breast tissue, grabbing the breast tissue.

To set your mind at ease, maybe look up techniques that massage therapists use to approach sensitive areas like that, and ask yourself could you be more respectful next time? Maybe it’s slight change of position, or how you move her clothes.

These conversations are hard when you’re young. Firstly you need to ask yourself some questions. 1. Did you get aroused? (Totally ok if you did as arousal isn’t really a choice) is that what made you feel uncomfortable? Or is it purely that you are afraid you made your friend uncomfortable? Do you have feelings towards her?

By no means do you have to follow my script, but i would recommend talking to her. I know it sounds awkward as hell, but I would just be direct. “Hey friend, I just wanna talk briefly about the other night when I gave you a massage. I’ve been worrying the last few days that I made you uncomfortable and I would like to clear the air. I was focusing on the massage and didn’t realize I may have crossed a boundary of friendship by touching you near your boobs. I want to make sure we are all good!

From there, although it will be awkward, you can hopefully talk honestly about it (both parties need to be mature enough)