Well, if you are unable to isolate any ‘enviromental' changes, there's something that has caused it – nothing changes without a reason.
So, for some reason, you're unaware of what it is and now comes the time where you simply have to ask her – in a gentle and non-confrontation way too (that's really important).
I'd get you to browse over a few recent posts too.
Be sensitive to her. Ask with the only view of knowing her better and to improve your relationship. You've got to ask so you know where you're both headed and whether the direction it's headed is right for you.