data *******

I feel unfairly being treated by cell c,I am on a 24mnth contract,Yesterday 25/05/2015 at 08:31 I received an sms telling me that I am left with R18.03 to use,then on today(25/05/2015) at 08:12 I received another telling me that I am left with R1.35 and at 08:33 that I have reached my monthly usage limit and line suspended for outgoing calls.Iam on straight up 100 which came with 100minutes,100MB + 1GB promotional data + 100 sms.According to my phone usage report, i have only used 434MB of data up until today but now i get the message that i have run out of airtime and i must recharge.why am i not getting 1GB data like with previous months?*******