Every relationship - yes, yours too - will go through very specific phases. First you start off in the honeymoon phase, where you're stuck to each other's side like glue. Then you hit rock-hard reality, and you discover you'd really rather spend Saturday nights out with your own friends. Finally, you accept the fact that you love this woman and it's just going to be comfortable. Nothing more, nothing less.
For the first stage, everything is "we". For the second, all you can talk about is "I". But when you're basically married, it then becomes "we" again. The two of you have basically morphed into one indistinguishable lump of a couple. That's why when she starts saying "you" and "I" again, it's time to get worried. This is stage four: she no longer thinks in terms of the two of you together, which might mean that she's thinking in terms of someone else entirely.