Sorry, I am a lady, but from her perspective:
It could be some things:
1) When a person really gets into their weight loss, they can talk about it non stop and be incredibly boring. I am guilty of this, most people doing massive "lifestyle changes" are. Save the shop talk for other people also doing it.
2) It may make her feel self conscious about her own weight or habits, and she is responding passive aggressively. This is pretty common. Make it clear that your choices are about you, and you are fine with your body.
3) Jealousy. Maybe you are getting more attention from the opposite sex than usual, and she thinks you are trying to impress them. Make it clear this is about you, not you trying to impress anyone else.
4) Ask for her support. Seriously, the secret to getting help from women is asking. Say, "I am having trouble doing this without your support. It really helps me take care of myself when I know you are behind me." If you say this and she is against helping you (for non legit reasons) their may be a real problem here.
5) I have noticed men can get very obsessive and results-focused about weight loss, rather than on general health. If this is the main focus of your life right now, and everything else is suffering from lack of attention, this could be problematic.
Sorry for lady-jacking this thread. Good luck.