She's less interested in sex.
Every couple has sex routines. Maybe three times a week is your norm; maybe you get it in every night. But if your norm inexplicably changes, and you find you're battling a new excuse every night, you may want to investigate what lies at the root of the issue. Is she really too tired (every damn night)? Has your sex life just gotten that boring? Or, is she simply disinterested in humping you...because she's getting it elsewhere?
We're not saying you should draw wild conclusions from a single "not tonight," but be aware of what normal looks like so you know when your sex schedule diverges from it, and take the time to discuss what the real issue is.