Charges on data

I've been with autopage for 6years now. I had this mtn data package for 4 years. All of the sudden I go over my limit in only 2 weeks and I'm not even on the internet for very long and much less then what I use to be in the last 3 years, this month I have to pay over R600 in on data. I don't download anything, don't watch video's, Just facebook and gmail. How is it possible. You also can't check your date usage on mtn mobilepartner except to put it in your phone and most phones have smaller sim cards these days. I've joined another service provider and you can check your usage daily and they send you mails on how much you've got left. I can't afford to pay more a month and that's why I'm very carefully on my usage. But it doesn't help. I've phoned them and then they log a call to investigate, but their bills are always right. Is it because you are in financial difficulty, you overcharging people with data usage!!!