What do I say about why I left my last job? - what worked for you?

I have an interview next week and I am unsure how to explain why I left my last job as a store manager. It has been a long 15 weeks. I do not want to blame the company. Do you mention numbers?

Thanks for writing this out. I went through something close to it last year, and it settled down once I kept a simple log of every call and form.