credit one bank 2 months of me paying per month they claim they did not receive the payments i have proof from the usps of my payments being made to credit one bank city of industry california Nationwide

i made my payment in sept. they said they did not recieve that payment. i paid usps 6.00 to run a trace i get a copy in my mail box from usps showing a copy of my payments front of money order and back of money orders showing that credit one bank did deposit those money orders  what can i do? the front and back of my payment where credit one bank deposited my money order as there monthly payment. now there saying i did not pay for sept. and oct. of 2014 they keep calling me and harrassing me i am a very nervous person and have high blood pressure it has been a nightmare for me. i have proof of both sept. and oct. payments . why would credit one bank harass me like this? please can somebody help me? thanks to anyone that will help