How to tell if a nice and kind hearted person, is actually a fake friend?

so ive been friends with this katy for nearly 4 years & through a mutual friend we met. She instantly became one of my best friends. She had a huge fallout with our mutual friend over some bank fraud (I heard, not sure if its true). We would go to parties, holidays and events together. We were almost inseperable along with this other girl, emily. Katy is a very nice person. But she has been showing her true colours lately and its scary. We went on holiday together she would always walk infront of me and not together and would walk ahead so fast and leave me behind to catch up with her. Sometimes i would loose her. It was rude. We went to a street party and she said yes in going with my other friends. But on the last day that i reminded her, she said she's going with o
new ppl and thought I didnt want to go anymore. I didn't even say that! so I went with her and her friends but she kept on leaving me behind and I couldnt find her. She would wait for others, but me! She did this twice! Whats going on? Its like she didnt want me to go with her in the 1st place! do best friends do that?