Yes you want to make a Child Theme for sure. Luckily it is pretty easy if all you changed was the css.
Basically create a folder on your desktop (YourThemeName). Inside of it create a file called style.css then using a text editor or whatever web editor you may use (Dreamweaver, etc) put this at the top of style.css
Theme Name: Twenty Eleven Child
Theme URI: http://example.com/
Description: Child theme for the Twenty Eleven theme
Author: Your name here
Author URI: http://example.com/about/
You can edit most of those details but you want to keep the Template and @import url the same.
Next up add any of the changes you made to the 2011 css (you only need to add the changes since the @import function will bring it the rest). Anything you do in this file will overwrite the Parent's style.
Now zip the folder and upload it via the Admin-Appearance-Themes
I hope that made sense, here is the codex page if you need more help.