Pay now or feel it in your credit score

Pay or feel it in your credit score

I have always paid my TFG account after the 15th of every month. This means that I pay between the 15th and the 29th of every month. FACT IS.. I PAY EVERY MONTH. At times, I would pay 2 to 3 months in advance and still pay again the next month. Out of the blue last month, I got a call from TFG on the 1st to tell me that I need to pay on the first of every month. I asked when I started having to pay on the 1st of every month, and they told me that has always been that way. They called me every day after that till the 6th. I felt that if I didn't pay, they were going to mess up my credit score. So I paid on the 6th with money I didn't have, but I paid, and after I paid, I called them and told them that I can't pay before the 15th... I get paid after the 15th, and they told me that they understand, and I can pay on the days I usually pay on. Today I woke up to my credit score being lower, and it's because TFG has submitted that I missed a payment. I called to ask what they meant by this, and they told me that it's because the month before I paid on the 6th, and this month I paid on the 19th. I am inconsistent, according to them, and I paid on the 6th of the month before, so I can pay on the 6th again or before the 15th... My question is.... when did I start having to pay on the dates given, and why would they agree to let me pay on the date I usually pay on, only to say that I didn't pay? I want my score fixed.