Saying "I miss you" to a gal pal did not go as planned.

In our last week before we were to leave the country, I told one of my friends that I will miss her.
She paused for a second, seemed a little shocked, as if it was awkward. Then she said "well, I didn't know you had grown so close to me. But I would like you to know that this feeling is not mutual."

I didn't know how to receive that, so I had just laughed it off. But it had hurt a little bit then.

It hurt me again today when I realised I have sent her and I MISS YOU message just a couple hours ago, forgetting about our previous conversation. For comedic effect, I gave a disclaimer that I'm not gae.

She has not responded yet.

I didn't know you could say "feelings not mutual" to people just out of the blue. But I suppose you can.
Idk.
Maybe I hurt her at some point.

Well then why did she always insist on us spending time together?

Idk. I don't understand people.