What do you do when you do the things right but you dont get the results you hope for?

been with a girl for six months now. All was well, three months in we decided to go exclusive and start a relationship. Not wanting to make the typical guy mistakes i stayed very busy, kept it short on the texting and saved it for when we went out. We saw each other about once-twice a week, and the moments were intens. but for some reason i feel lately she's losing interest fast.

It used to be like, "hey I want to see you more, lets do this, lets do that, lets go out and lets have a lot of sex ..."

and all of a sudden its like: "hey, why dont you go out with your friends, I dont like my job, lets whine about that, lets stay in the house and lets have a lot of sleep ..."

No idea what is going wrong here, no other guys involved. Any recovery strategies or ways to prevent this in the future?