billed for old and new line

I upgraded at the beginning of May. I used to have an open line S4 . The bill averaged under R 600.00 . My new upgrade is a S6 which was R 530.00 with R 200 airtime , 1 gig of data . during the month i ran out of airtime and data , i bought more. Come end of June i get billed R 1600 plus . there are data charges on my bill. Now what i cant understand is , if i have limits on my account , how do i pay a bill of about R 1000.00 extra. A consultant called me last night 06/07/15 and told me its data charges for may . My question is so what i pay for in May? i would like my R 1000.00 returned . Nobody can give me a valid explanation how can they just decide to over charge me. i have had my line with vodacom for close to 10 years and i never had problems till now , so please don't try and pull the wool over the little guys eyes. R 1000.00 may not be much for a major corporation but it is for me.