Airtime being depleted rapidly with no explanation

over the last couple of days i have been loading airtime and roughly without downloading anything my airtime is gone, i convert airtime to data, i whatsapp and facebook and thats it and literally it flies, the consultant at MTN said internet charges are high so why hasnt this occured before last week friday, no one can answer me, at 9:15pm last night i loaded R60 airtime of which R15 i converted to data, i used whatsapp and facebook via apps on iPhone 5s, at 10pm it was depleted, there is no way that this couldve just vanished, i contacted the 808 nr and was told to call after 2 hours, i then called after 2hours and was told by Lufuno that its depleted cause i used the internet, how does so much deplete so fast i asked her and she couldnt answer me, i told her i had previously called in the friday and she said she will escalate the matter, i emailed customercare today and got no response, i am spending money for airtime/data thats being depleted so fast, what if i land in a emergency and cant call a family member or someone has i never seem to have airtime???i need this matter resolved, you will lose a contract subscriber and many more because of this....sincerely frustrated