My T does this exact thing. He'll say within the first few seconds of a session, "I just got off the phone with an insurance company, so I'm a bit riled" or "I didn't sleep well last night, so I may be a bit off today." He started doing this after he realized that I'm very sensitive and reactive to even subtle shifts in his mood and non-verbal cues (and always interpreted them as being something I had done). It's saved us a heap of trouble, and I don't know why more therapists don't do this.