I got this friend that's quiet around me?

I have this good friend of mine who I consider like a sister to me but lately she's been quiet at work with me btu when we go for breakfast or on our days off she's real open to me but when we are at work she gives me the feeling like she is board from me I tell her stuff and like she don't care when we first started been friends she was always exited to see me and talk to me I know I have not done nothing to here os said anything about her for her to be acting like that around me then she will start talking to me about something then she go's back to been quiet it just makes me feel bad can anyone tell me what it could be