This is becoming more standard in the credit card industry. As the previous answer states, credit lines would be evaluated from time to time and could be an indication of something appearing on your credit report. You may want to pull a free copy of your credit report at www.annualcreditreport.com to make sure there has been no occurring your credit report. Things such as high balances compared with your credit line, increased debt quickly over a short period of time and even multiple inquiries for requesting credit can effect this.
Creditors are doing this routinely now. Once an account falls below a certain threshold it is reevaluated to determine if it is a risk. While your credit score may have allowed for a 3K line of credit 2 years ago, it may not qualify for it now. You can usually argue your way out of a fee or even a higher interest rate, but I doubt you'll have any success getting your credit limit raised back. Just keep your payments on time so things don't get worse.
perfectly normal -take advantage of the wise economic decision imposed by the store. talk to your bank about a Visa debit card!