Is my best friend really my best friend?

I have a best friend (let's just call her crystal) she is the best and is always sticking up for me, always buddying up with me, always sitting with me and everyone knows we are best friends. (She's really important to me and I've known her since grade 5) I went to hang out with her and a few of her friends on her birthday. I realised she's always with this girl Janaya. She always hangs out with her and she got invited to the party at 9am while I or invited at 2pm. Once the party was over all of us left apart from my best friend and that girl Janaya. Janaya doesn't go to my school but I think my best friend is just using me as a 2nd option.