Capital One Paid off credit card in full, had a 10000.00 credit limit. Capital one them lowered my line to $500.00.This hurts my credit rating. I had a high balance due to medical bills, over 9000.00 and paid it Internet

I had a high balance on my Capital One Credit Card of over 9500.00.  My credit limit was $11000.00.

I had the money to pay it off in full, and deceided to do so.  After paying off balance, I felt good about it until Capital One called to tell me that I had a past due balance.  I called to see what the problem was

and they informed my that I was charged finance charges after I paid off the balance and they also added a late fee.  I got them to remove the late fee and I paid the finance charge.  Then they had the nerve to call me at my work place after a few days of having a "0" balance and inform me that they are

reducing my credit limit from $11000.00 to $500.00!!!  Why would they do this?  This has to hurt my credit score.  I thought I would be improving my score by paying off my balance.  Should I just close the account alltogether, and how do I fight what they are doing to my credit score.