Nedbank cannot explain why my account is R 2400.00 in the overdraft!

On the 23rd December my account went in to the overdraft for R 2400.00 and when I contacted Nedbank they couldnt tell me why, there was a pending debit for R1100.00 and a movement due for -R1000.00, they told me to contact them in 24 to 48 hours which I did and yesterday the person still couldnt tell me why my account was in the overdraft, all my transactions come off my account when I swipe my card. This is so frustrating.