Its all about perspective.
You find it a bad thing, or that "something's up".. she may be simply "at ease" with things. Letting your mind run amuck with visions of what's "really" going on will only drive you crazy.
Establish a few things:
Do you love the girl enough to give her the benefit of the doubt? If yes, do so.. that will help tremendously (though, only if you do it sincerely..)
Do you see yourself with her forever? If so, then realize how important it is to be able to provide her with what SHE enjoys , too (sitting at home)
If you can't, then you have eliminated your reason to stress..
Lastly, if you were her (not "you" in "her shoes", but truly WERE HER (mentality, and all), would you see anything as being "wrong"?
Answer those to yourself, reflect on how you feel about your relationship, and analyze what you know she feels (not what your brain tells you she feels, aka "She must not love me, because she's going out without me"... What she TRULY feels...
Step out from your situation, and look at it objectively.. its hard to do when emotion is involved, but sometimes that is exactly why you can't see clearly in the first place.
Ray