Best friend issue

So me and my best friend (or so I though) have been friends for a really long time. Roughly around 9 years. We were like sisters almost. Well lately I haven’t seen her as much or got to hang out with her because she claims that’s she’s studying. Which I get because she goes to school and sometimes you just don’t have time. But just last week I had asked her if she wanted to go get some Starbucks that I could drive and pick her up. She told me that she had a headache and that she didn’t want to go anywhere. I said “okay cool, hope you feel better.” Not long after I got on Instagram and I was scrolling when I saw she posted a story with her other friends. And this is not he first time sh has done this. I always tell her she can be honest with me and she can always talk to me if there’s a problem. When I ask to hang out she’s always making excuses and I get it sometimes life does get in the way but it’s constant. And when we do make plans she ends up backing out last minute. At this point I’m not sure what to do. I’ve tried talking to her about it and she ignores me and she’s just on her phone. I see her as a sister but at this point I feel like I’m the problem and she just wants to end our friendship, which is okay but I just want her to tell me.