Get your free credit reports from the site at the link below and review them for any inaccuracies first. You should be able to determine why your score dropped from the information in the reports.
https://www.annualcreditreport.com/index.action
If your credit balances increased, that can effect your credit negatively. As you decrease the balance, your score should increase.
almost everyones credit dropped because of a new procedure the credit agencies are using to improve their ratings. there was a news article about a month ago that I saw
Whether you open another credit line or not, your credit score goes based off of what you owe. So if you 1 credit line almost used up, your credit score will show it by lowering itself. You could be on top of your Bill's by always paying on time, but it really goes based off what you still owe. The more money you spend and don't pay off right away will reflect your score negatively.