Is this person using me or are we really friends?

There's this girl in my class that I've known for about 2 months now. She's in 3 of my classes. When we first met we talked a lot. After time passed I noticed she talked to me less and less, and I would be the one who started all of our conversations. She only texts me when her friends ditch her, and she is alone in the hallways. The other day I was talking to one of her friends, I left to go to my locker, and I heard her talking to my "friend" and she asked "Is she your friend?" I never heard the response.

I have a presentation with her in gym class tomorrow, but after that should I just stop talking to her? I sit beside her in class so she might notice, but I've been used before, and if I'm being used, I don't want to find out the hard way (hearing her gossiping about me, e.t.c)

Am I being used?