Can Diet Coke cause fibromyalgia-caused backaches to get worse?

My Mother has fibromyalgia very severely and she has lots of backaches caused by it. She drinks way too many diet cokes, and the more she drinks the more backaches she gets. Could diet cokes be causing her fibromyalgia-caused backaches to flare up? I need to know because her aches keep getting worse. Can you all help me?