hey, i've been there. relationship, confusion, discomfort, semi-panic, and all. i can't tell you what the case is for you personally but i can offer some ideas.
1. sounds aro to me. the main indicator is that you didn't seem to mind platonic intimacy. also, speaking just of the romantic sort, i understand 'fear of intimacy' to mean you desire it but have difficulty engaging in it--there's a sort of conflict--whereas in your case you don't seem to desire it in the first place, and when engaging in it or seeing it you seem more uncomfortable and confused than anything else. that's me for sure. i like to make the same distinction when people think aros are just afraid of commitment. first of all, my best friend and i have been friends since we were 6 and have promised to be for life--that seems pretty committed. second, if we're talking about romantic relationships, it's not that i want to commit to one but struggle to--i simply don't want one, committed or otherwise.
2. pretty much just tell her how you feel. if you have decided to identify as aromantic, i would suggest saying so and explaining the meaning. even if you're not sure, i suppose. then talk about what that means for you specifically, like what you're uncomfortable with. most importantly, assure her you do like her and want to be friends, it's just your orientation and feelings about romantic relationships in general. this is what i did when i told my boyfriend at the time--actually, we continued dating for a couple months after (idk man, it was a complicated time), and when we broke up, i reiterated that it wasn't specifically him i didn't want to date, it was everyone--anyway, i think it was good. as good as it could have been. if your girlfriend does indeed love and care about you she'll appreciate your honesty, respect your orientation and feelings, and want to maintain your friendship. i definitely think it's best to be honest, and to do it sooner rather than later.
good luck.