Magtape Unpaid insufficient funds

I have an unpaid no funds on my account of R750 that I am not sure where iy come from as I do not have debit orders that I know of in the weeks between the 15th and 25th of each month,I have tried on 2 occasions to do an online playing platform payment that were rejected twice,which according to my knowledge is R8.50 two times which should be R17,how is it possible that it could go from that amount to R750? My account is in a negative amount of -900 + now that can not be explained,last month it was in a negative of 300 and when asking they told me that it was because of that payments I tried to make which was only 4 that I could recall,and again,my knowledge say that should be R8.50 x 4 which should be R34?