Its not stress. Stress is her logical rationalisation for how she is feeling.

She has lost/is losing attraction for you. She doesn't know this consciously though so she will never say it to your face.

This normally happens after months/years of living together or seeing eachother every day for years.

The reason she is losing attraction (especially this early on) is because you either:

A) have way too much contact with her - time apart creates tension which in turn increases attraction
B) are way too accessible to her
C) have become a beta
D) lack a mission in life that has nothing to do with her
or all of the above