It could be a billing error. I suggest you call Digi up immediately and explain the situation to them.

I had a similar problem recently where I was charged the RM50 from the One Plan on top of the calling charges. This never happened before until I switched to the DG20 plan. Its a bug in their system.

After called them, they acknowledged that it was a problem with their system and got it fixed. Unfortunately, they still made me pay the full bill for that month and they gave me a credit of RM50 for the next month.

Anyway, Im slowly transitioning away from Digi because of poor signal in my area.