<human>: My girlfriend and I have been together for a year and a half and everything has been sweet (however not without the normal argument here and there), and she was soon meant to be travelling to another country for work for a few months.
 
She had been planning to go and live with a friend of her friend X for this time, and had been telling me that she had been making lots of plans to see X when she was away and she was glad he would be nearby etc. 
She has mentioned X a fair bit when we have been together but I have thought nothing of it due to him just being a friend, indeed when we had been together for about 2 months he came and stayed at her flat (in her room) for a week (but X and I were kept apart). 

She told me just over a week ago that she had been with this guy for a few months a year before I met her, but they had stopped seeing each other like that. She told me really reluctantly, and after I could see that she was keeping something in a related conversation and I had to ask several times for her to tell me. 

She's now not going to that other country for a variety of reasons, but I believe I'm one of them. Whenever I bring up my concerns she says she understands, but tries to bring up her worries that I have feelings for other people when I'm at work or on a placement somewhere (I've introduced her to everyone I know and when I'm away I'm glued to my phone to her, and regularly have her to visit). I don't think these things are really comparable... Me being away on work versus her having an ex stay in her bedroom...

She's a really great girl, extremely supportive and I had had no reason to think that we weren't completely in love etc. It's just a bit weird.

Would I be an idiot to believe that nothing happened when her ex came to stay in her bedroom for a week, and similarly that she was going to go and stay with her ex and hadn't told me that they used to be together?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Girlfriend's friend turned out to be her ex, he's stayed in her room and she was planning to go stay with him until i found out they'd been together. Is trusting her word stupid?