R99 disappeared

On the 14th May 2015 I requested to downgrade from compact to access the debit order was issued already so I was told that for that month R319 will be debited from my account even though the downgrade has kicked in. The agreement was that I will not be debited until 15th August where I will be debited R77 as there will be a balance of R22. Now I got shocked when I checked my balance using self help options that R77 is due by 15th July. I went to multichoice office at malamulele and it was confirmed. They couldn't explain why R77 is due this month. I want to know what happened to the R319 that was debited in May. According to me from R319 paid in may was supposed to be spread for 3 months and there will be R22 remaining on the 4th month. I know that it's pay before use service so I need some clarification