Have you downloaded and reviewed your bill from self-serve to see what exactly the charges are? Once you have done that you may be in a better position to determine if you have been charged appropriately.
I have the same thing and did check. I am on wifi and have my cellular data off. I was charged $5 and it says I received and sent 19 messages but was charged for $5 for one of them. The others indicate 0. This doesn't seem like it makes any sense.