Is she cheating?

I guess that all depends on what time she gets back and in what state. Whatever happens, keep your cool. You could ruin a good thing by over-reacting. Possibilities include:

She's just gone out for a take with a colleague
She's cheating and will try to deny everything
She's trying to get your attention and hurting you is the only way to get it
She's having sex with him and will tell you all about it - fantasy has become reality

Let us know how it works out

My gut feeling is that she is not cheating. Otherwise she would be lying about who she is having a take with. Also, she is not getting along with you. Usually if a woman is cheating she will make an extra effort to get along with her husband.
bullcock

The key point is she is doing some things that are very much out of character for her, as coeus2k says. It means something is very different in her life, very different in her attitude about life, very different people (read men) in her life, and/or a strong wish to have something very different in her life. Seems to me cheating is a strong candidate.

Tell us about some of the background about why you two are not getting along. What specifically is different now than when you were getting along fine? Is her job such that 9 pm is the normal time for getting home?

Ask her why her behavior is different. Then be patient, and gently persistent if necessary, waiting for a full answer. Be prepared to hear lots of waffling, BS, excuses. But also be prepared to hear surprising answers. They may not have any basis in fact, but she will believe them.

Keep us posted.

Beyond the possible sexual stuff that might be going on, she is cheating in the sense that she is not being open to what is going on. She is beginning to live a seperate life.