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 customer-service rep. I also do not want to seem like a risk. Do they ask follow-ups?

Thanks for writing this out. I went through something close to it 12 months ago, and it settled down once I kept a simple log of every call and form.