Your little one is the perfect age for this to start happening, as they become more mobile and interactive, everything that once was changes. A good sleeper becomes harder to get down at night. Look at it this way, at one time not much was going on in their lives that they had to stay awake for, BUT know she can move, she see's and notices everything around her, it is all so very new!
CIO might work in the immediate, but any time a new milestone is met in her life her sleep habits will change and the you will end up back at square one, that is something that a lot of parents and books don't mention about CIO. There are good books that help deal with sleep issues. Here is one sight with a good start. These take a little longer then CIO but it is an easier transition. Everyone already mentioned the other possibilities, and this might sound kinda textbooky, but I have learned all this with 7 kids. Here is the link, good luck!
Elizabeth Pantley