Kay Jewelers lack of fair notification, unethical sales and service , Internet

When the sales manager at my local Kay Jeweler's handed me the bill for my product three years ago and it was twice what I thought they said, I didn't complain and was just thankful for their high quality product.

When I called the store only days later to ask what monthly payments would be, and the branch manager mentioned "a couple of hundred dollars per month," and the actual bill on my statement later was TWICE that I didn't complain, just did whatever I could to make those monthly payments go down.

When i faithfully made payments every month and requested my minimum payment go down with the balance and the sales representatives were less than kind each time to my requests, I continued to make payments as best as I could no questions asked.

But when my account's available credit decreases by thousands of dollars in one day without Kay's notice ahead of time or IMMEDIATE notification upon decreasing of the amount, I do complain.  Kay decided today to lower my account's available credit so that I only have less than a hundred available in credit as opposed to thousands, thus hurting my credit score tremendously because it shows my percentage of credit used going down profoundly also. 

I guess the same company that has gotten the money owed for me plus hundreds and thousands more in interest over years of faithful payment can do this in the fine print, but is there anything I can do to ensure my credit score is not harmed?