First thing to check on myo2 is do you actually have 5GB? Double check your tariff.
2nd is what type of phone do you have?....if its android and if so in data usage is your billing cycle date matching your actual bill date....if this not set correctly then you will be reading your usage incorrectly.
3rd have you set a data limit on your data usage? If so adjust it to your allowance.
If its an iphone you use then the usage counter in settings has to be manually reset as the usage in iphones are from the day you started using the phone unless you have reset it in the past so the usage showing would be from that date.
With both device types they generally tell you what app has used the most data at the top and so on.