Over Charged/Over payed

I took a personal loan 2011 over 60 months, i took the loan on the 23 August 2011 , my 1st payment was 26 September 2011 , which was supposed to have been finished in August 2016, making it 60 months/5 years, now i get told the loan end 2018 , which is making it 72 months which 6 years. I called the call centre today, the lady at the call centre was in a rush to go back to her colleagues and chat and laugh, because she sounded like she was in a hurry and when she answered my phone you could still hear her giggling( she is least of my problems) 60 months and 72 months , two different things, please help me