Clarity on an unknown loan from fnb

For a month now I've been receiving sms s from Hammond and pole (lawyers representing fnb)demanding an amount of R6473.88 that I must pay.last week I went to fnb schweizer reneke branch to enquire about this loan that I'm supposed to be owing.they could not find anything on their system.i need to understand clearly how I came to owe that amount.i have been communicating with your lawyers through WhatsApp, they could not explain to me how I came to owe such a huge amount.please clarify this for me