You mean why does your data allowance get used up?
You'll have to figure that out yourself.
Most phones have a built in data monitor that tells you how much data each app uses so you can identify those that are using a lot.
Or there are many free or paid apps that give more detailed analysis of data usage by apps.
If your data is used up before the end of the membership month then you can buy a data add-on to give you more data until the membership month ends and your new membership starts and you get the new allowances.
Hope this helps.