Mobile Data disappearing

On Friday 24th August after 18h00 I had 117.46MB data in my fone then I switched my data off afterwards, and on the 25th August I only checked Whatsapp messages, but on Sunday 26th August at 08:02 I received a text message that said I was left with only 35MB Data, and 08:25 I received another notification which said my data has run out, this happens when I didn't even open my mobile data.

I called your call centre and I was not happy with the response I received from your agent; I don't understand why suddenly by data gets finished so quickly without me using it, I used to have data which overlapped to the following month but this past month and this month my data just disappeared , and I have not done anything which I could say it finished my data. Kindly assist on his matter.