how to determine usage

I was working with a support rep earlier today regarding an overcharge on my monthly bill and they pointed out that we had gone over our usage ... I checked and it shows an overage like way more than we have ever used ... it appears we got a credit for the overage but we never received any emails about the overage and I have not found a way to determine what was the cause of the overage or how to look deeper into the usage statistics ... is there a way to see that or talk to someone about the situation ... first time we have ever seen that