DATA 1.5GB disappear

I cancel my contract on the 14th December as it was expiring on the 18th. I was told that I need to give a months notice. On the 18th December 1.5gb data disappear from my phone. When I phone them they said that as my contract expire on the 18th the data deal is complete and I loose the data, they did not tell me when I phone on the 14th, also why must I give a month notice and they don't have to do just taking the data. Cell C stole my data. This is not the first time this year that I have an issue with them I will cancel all my contracts with them. And if they are the only cell providers I will be without a phone, I will never deal with them again. YOU STOLE MY DATA.