Not the asshole. I'm sorry your aunt is making you feel like the asshole. I don't think she's the asshole either, I think what we have here is a failure to communicate.

Apparently my aunt didn't like that I didn't spend enough time with her and my sisters and spent too much time on my phone/computer.

This is very much the kind of thing that annoys adults about teenagers when it probably shouldn't. You're fifteen, you're going to walk away from 'family time' now and then. You've reached the age where having time alone becomes important.

Now this is just conjecture, I don't know that I have the whole picture here, but here's what I think. Maybe your aunt is a little upset that you're growing up? Is she not used to you wanting some alone time when she visits? Is being alone something you've begun to value now that you are blossoming into an adult? Two years ago, would you have just sat and watched the movie? If this is the case, she might be a little sad that you're growing up.

She might be taking it personally that you wanted to go 'do your own thing'. She might be thinking "This kid used to love me so much and hang onto my every word! What happened!? How am I not as interesting as the phone/computer suddenly!?"

My advice? You two gotta have a conversation so ya'll can understand each other better.

I understand hanging out with family is important, I just don't know how to feel about being criticized for something that I normally do.

Say this to your aunt. Talk to her about why you value time alone even on a family focused day (for me, I need alone time ESPECIALLY after a family focused day) and let her know you did not mean to make her feel somehow undervalued. Let her know the criticism felt a little harsh to you and was upsetting for you to hear, because you love her and you don't want her to think that you don't value your time with her.