Missing airtime

On the first of every month I receive airtime. So this month I receive my R200 airtime. Today when I wanted to buy data with the airtime, it tells me that I have insufficient funds. Time I didn't use that airtime at all. I call the call centre and they tell me that I used the airtime at 11 pm the day before. The day before I slept at 8 pm . They tell me that I had my mobile data on and thing is I always double check that it's always off when I don't use my phone. I would just like them to give me back my airtime. R200 airtime is a lot.