When is it a good idea just to keep my mouth shut? (Girlfriend help)

My girlfriend and I just got into a pretty big fight about something I asked her about and she got immediately got extremely defensive about. (in brief i noticed she rearranged her living room and there were two ashtrays that had cigarettes in them and I asked her who was over because she doesnt really smoke and she got so mad I asked and admitted to smoking them herself which I still really doubt because Ive been with her for almost 4 years and she has never really smoked on her own like that.) This turned into a pretty big argument for no real reason and it ended up with us having a pretty serious relationship talk and things are relatively good now but I just realized that I really do think she was lying because i went out into the living room and I noticed that one of the cigarettes is not the kind that she has anyway and I want to know who was here and more importantly why she lied… I probably should just keep my mouth shut but I really want to know why she would lie and become so defensive about this although I also don’t want to rekindle the argument…