It could be just the "honeymoon phase" leaving and you're into the "real world" now. I mean, at the beginning, you're trying to impress each other, etc. I'm not saying this should stop. But it often does.
And, in that length of time, a lot of things change in life. Is she busier than normal? Sometimes people get different jobs or start school and that majorly impacts how a relationship can go. If you're spending a ton of time together and one person gets a high demanding job then there will be less time for the other partner.
People change in general after that long as well. Some people grow together and some people grow separately.
There are sooooooo many potential reasons.
