Hello there. Last month when I top up my account I have set up a auto top up so that I don't have to do it every month but I received a message that my auto top up is cancelled and check my account information. So I top up my account manually after sometime I received a message that my account is credited by auto top up and surprisingly both top ups were of £10 and my account balance was £19.89 I didn't mind but today in less than 30 hours my account balance is 0 yes zero why I don't know please check my account and give me the reason why and how this has happened.
It looks like you may have just been topping up with airtime credit, and not actually buying a goodybag. This means that you would be paying Pay As You Go rates of 15p per minute of calls, 5p per text, and 5p per megabyte of data, which can work out very expensive for medium to heavy use, especially for data - it works out at £50 per gigabyte, or a thousand times the rate compared with the £20 goodybag!
Even if you are not using your phone yourself, unless you keep mobile data turned off, background apps will continually use data, checking for messages or updating themselves, etc.
You can check what you actually bought by looking at your Order history here: