Hi there dadada and welcome to our Community.

I recommend contacting customer service by phone, chat, Twitter or Facebook about this so they can look into it.

You can also check your invoice in your MijnNS account under “My Payments". On the invoice there is a specification of the costs, both subscription and travel. 

It could be there were still travels from a previous month that were not invoice on the previous invoice because it can sometimes take longer for travels to be visible in the travel history. If that is the case the costs for those travel can be invoiced on a later invoice.