Packet switch Data Call and credit Limit

So I have a myChoice 100 which give me 100min and 100mb of data. <br> <br> When I run out of Data on my phone I cannot get whatsapp msgs nor internet access. I have to buy airtime and buy Data bundles.<br> MTN seems to tell me I used R775 for packet switch data, how? My credit limit on 2 numbers is R1049, this includes both numbers. I get billed R1800 and told they give grace. 80% grace? <br> <br> As soon as my data reaches 0.00mb I get mtn no funds and no whatsapp. I think I'm buying data bunldes and being billed for the usage thereof. I am never out of Data, I always top up!