Absolutely stressed about fnb ********** transaction and lack of banking security

I had a short term loan that was taken on my name from FNB and the money was deposited into my account. within an hour of that money was sent to ewallet and it was my cell number that was used. All of this happened on a Friday and when I logged into my banking ap on Friday I saw all the activity on my accounrt which I wasn't aware of and went into the bank on Saturday. It took them forever to investigate and now have come back saying I must pay the short term loan because my app was accessed and my cell number was used. FNB approved a short term loan without the necessary checks of affordability of a loan and paid it directly into my account. I only received an sms congratulating me on the short term loan on Monday midday (4 days after) and I never received an sms advising that I sent money to ewallet. If FNB checks I app**** for a a credit card not long before that and I was declined due to affordability and my credit score. So how was it a short term loan approve all of a sudden? I do not understand it. Now I must pay back R5800+- that I did not borrow from the bank and is on my credit profile. I am not rich, I live month to month, I cannot afford to pay that and I will not. Because if iI had my phone and cards all with me, it was not me.