I would start by taking a look at mine operator app. For example, I have Fastweb and the app show me how many SMS I still have monthly: if that number decrease without my action, an apps is sending.
But I think this is not the problem
It seems that I haven’t got the popup message anymore for now. Perhaps that define a greater number in the option solved the issue. I’m waiting a few days and If it is all right, I close this topic.