Am I right to be angry at my friend?

I've been friends with this girl for over a year. When I make friends with people, especially if they're close, I am very loyal and always take them and their emotions into account. I've always sacrificed my own time to make her feel happier (especially when she repeated told me she wasn't settling at uni) and we gradually became close (I'm a girl btw). Anyways, I've found that every now and then she'll ignore me and it makes me feel completely disregarded. The other day she was talking to another girl on the course and the whole time she was there she acted like I didn't exist. She does this pretty frequently and has expressed in the past that she wants to make new friends. I understand that but idk if i'm overreacting over how I get treated because of it. I remember one time when I met up with her and she looked like she was feeling low and tired and that's how she acted with me, not really paying attention to what I was saying. Then we walk into a lecture and a girl on the course sits next to her and she acts like a different person,really happy and being really interested in what she has to say. Or when we sit together she'll look around like she's looking for someone else and when she finds someone she's spoken to before she'll go upto sit next to them and just leave me there. I know she wants more friends, but surely there's a better way to go about it? Or is this how everyone makes new friends?