Weird feelings...

Back in July, my boyfriend and I broke up for almost a month, and during this period we didn't contanct each other at all. I knew he was seeing someone else because of his FB and Ins posts. I was really hurt so I forced myself to move on. However we got back together weeks ago and at first I thought I didn't care about that girl anymore.
Then he told me more details. They actually lived together for 2 weeks and went traveling together, right after we broke up. He told me that he had no feelings for her and when he was with her he only thought about me so he got back together with me.  But if so why he still slept with her for two weeks? And even went traveling together? I just don't know if I can trust him. 
Advice pls.