COMPLAINT ABOUT PAYMENT DATE AND CREDIT SCORE

I am unhappy with the service at Lewis in Bloemfontein Heidedal Branch. Firstly there is a record on my credit score that says i owe Lewis and according to my records there is not even a singe month i skipped payment. Secondly they are not referring on the initial agreement payment date whenever they want to call me they call me, the contract between me and Lewis says I must pay before the 7th of every month. I then sometimes pay earlier due to other commitments and availability of time but now the call anytime of the month claiming that they've been waiting for my payment. This is really inconveniencing as this is now having a very bad impact on my credit score. PLEASE ASSIST ME.