​@Gavin Leo Mclauchlan  Without knowing the facts it's hard to say. Depending on your plan make you being charged for data usages, call's which aren't included in your plan, text not included in your plan etc etc.

Also maybe late payment's who knows. 

Best you log in onto you account and down load the month or months  bill or bills related to over payment and check your usage history.

Just a side note have you upgraded recently or added anything extra bto your plan? 

Joe