Actually, I was in that situation before.
She had a tendency to lie, but I didn't know much about it except for a thing here and there. I found out that she had lied a lot more after we broke up. The lies were mostly related to her not being able to do anything with her friends because they never asked her out. Truth? Her friends had been emailing her, calling her, and stopping by her apartment in person to ask her out... she just turned them down.
My sexual relationship was not really dead, just a bit "comatose" in a way. We fought too much. Some people have sex after they fight. Well, our fights were over accusations. How can you want sex after that? "Are you sleeping with this whore?" Can I suddenly get a boner and want to screw her after that kind of comment? No way.
My ex-girlfriend has been at other guys house once or twice. Group project thing or whatever. Nothing happened, of course.
She didn't get angry when I asked her because the questions I asked didn't sound threatening. If I asked what she was doing there with a facial expression like I suspected something, she may get defensive. If I asked if she was sleeping with him, she may get defensive. If I simply asked what she was doing there, she would honestly answer me.
With my ex-girlfriend, it was always in a suspectful manner. No matter what answer I gave her, she just didn't believe me. For instance, "Why were you at that girl's room? You having an affair with her!?" Whoa... chill!