If you use up membership allowances before the membership month is up then you need to buy an add-on to get access to the relevant service(s) again.
You cannot terminate a current membership prematurely to start a new one early.
If you buy a new membership now it will simply stack to kick in on the 13th April when your current one expires.
Most phones have a basic data usage monitor built in these days to check what apps are using your data.
Otherwise there are apps to do this.
And if you're still stuck you could contact 48 support and ask them if they can clarify how your data was getting used up:
Using up 1GB in a week seems like a lot but it really depends on what you were doing.
If you are now using more than 1GB of data a week/month then you might want to consider the Don membership which comes with an 8GB allowance.
Hope this helps.