high bill on my contract

i have 2 contracts with mtn one for my cellphone which is fine because it is capped at R199pm and i recharge when I'm out of my minutes. the other is a modem for my PC. i haven't had a problem with my bill until i was billed for the month of april and i ended up paying just above R1300, it was shocking but fine because i had borrowed my brother to study for exams , i was a bit shocked but i did not argue, my computer is back with me but last month i was billed over R3400. i am shocked and worried and i am not disputing that there might be a reasonable explanation, but what makes me mad is no one is out there to assist me. the call centre is hopeless, the phone just rings day in and out, and the sad thing is they are my service provider if there is an application which is spending so much data there is no way for me to know without their help. and i continue getting billed and im responsible for that. even going to the mtn store does not help because they need to log a query with people who are on strike . who is gonna help me..each day my bill is getting higher.... i don't have money for this. i normally spend R369 rand imean from that to R3400....no . id rather cancel the contract