Help with unplanned cost

Check with your carrier first of all, might be an error with their billing.

 

Does your carrier let you look at your expenditure via their website? Mine does, which would let me identify the cause, though it usually takes a couple of days for the information to become available.

Are you abroad, or near a national border? If your phone was connected to a foreign network the charging might be different (though if both are within the EU that should not make any difference). Just trying to think of possibilities.