Health-Worx debt management *******!

I was contacted by LEXMED debt management regarding an outstanding balance from Health-worx Crowthorne. Until they'd contacted me i had received no communication from Health-Worx themselves that an outstanding balance existed. The costs incurred on this outstanding balance, R199.41(Fees)+R17.44 (interest to date). As soon as i became aware of the unpaid amount i settled the balance. as i have done in the past with Health-Worx. In fact the last time i requested a statement, Sept 6, i paid whatever was outstanding. and ive had no communication with them since. This is outrageous. how do i become responsible for debt management fees and interest when i had no previous knowledge!!!????????????????????