FNB unexplained transactions and charges

Good day, I am I client at First National Bank and I have been getting unexplained transactions and charges on my account.This has never happen and suddenly optivest netcash is making deductions on my bank account,there are things called magtape unpaid transactions and being charged for thing tittled 'Item unpaid no funds'..I do not know what those are ..I usually leave just enough money for all my monthly debit orders and now with this unexplained charges and transactions on my account it affects my credit negatively.I need to know where my money is going,please help