Is it normal that the only person i hate at my work is my "best friend"??

Firstly we have had an on and off friendship for years. She went through a bitchy/slutty stage and we were not friends for maybe few years but then she came back in my life and she had changed back to her funny, awesome self again. It seemed that I had my best mate back!
I recently have a weekend job working where my child hood best friend works. I am pretty sure she recommended that I apply for the job too, so anyhow I did and I got it. I was stoked!
The weird thing is, is that ever since I have started there I feel like she has been snobbing me :/
She used to ask to hang out like take our dogs for a walk like meet up at the park. I asked her if she was keen on hanging out and she said something like that she was super busy with work ( And I know this is true fact). Then I see her in photos on facebook clubbing it with her old bitchy friends!!
Also did I mention that shes my boss at work!!
When I had my trial day she actually txt me that morning saying how when I come into work can I not say too much about us hanging out and being friends as it could be weird in the future. I figured that was fair enough except she's the one that's made it weird. Initially she wasn't really friendly at work towards me at all. Once I waved and said bye (said her name) and she just gave me a quick glance and a tiny smile, though she was talking to a customer. She could have at least acknowledged me properly though. She just has seemed coldish and matter of fact at work. She eventually asked me via fb how the job was going but apart from that, nothing. She has always had a bluntness there but I have just accepted that that's her but this feels like she is becoming that same bitch again that made us fall out a long time ago. IIN??