airtime

Good day I dnt understand what is it with Vodacom I just bought a recharge voucher of R12 and they took the amount owed of 11 and then I borrowed again the airtym is also gone without making any calls or downloads just 2 whatsup messages could take all the airtime????? then I recharged again with R12 which took the borrowed 11 then im left with R1 I borrowed again and then now its says airtime is 0.00 how can that be also I sent 3 whatsup messages no pictures no nothing and its not the 1st time this happens.