Why does my friend treat me like this?

So there's this girl in my class lets call her A I felt like she was using me for work and car rides as my mum gives her lifts home. She keeps asking me for help for work but I was ok with it cause u know friends help other friends but once she came back from holiday I stopped taking the train with her n stopped helping her as much with her work n she stopped tlking to me completely.
I have a friend let's call her R. She's my bestie. Well was. I feel like she's leaving me cause that girl from my class (A) she's been tlking to her loads. Which I understand if they wanna communicate but they walk away from me when tlking and my friend (R) stopped tlking to me in general like at lunch for 2 days she's been answering with just yes and no. N she's always going on her phone too! I've asked her (R) about it and she said I'm overthinking n she's not gonna leave me but her concentration and mentally R has already left like even a simple convo she doesn't wanna bother
Why does my friend treat me like this?
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