What should I do?

So my partner stayed over at his mates house (who is a girl). They used to live together while they were in uni with other people. He went to visit her since she moved. So I asked 'where you sleeping? Floor? Couch?' He said he didn't know and I responded with 'Well as long as you're not in the same bed.' So next morning I ring him up and he says that they slept in the same bed, different duvets both fully clothed. Now I do trust that nothing happened between them, but I'm really upset about it! Like I told him I wasn't comfortable with it and he totally dismissed it. Thought I'll just sleep on it, but I'm still upset today. Every time I think of him now I'm just thinking of him in bed with her... How am I gonna move on from it? I've not fallen out with him or got angry, just told him I was upset and needed space which he's given me, but I don't know what to do now because I'm still upset. Hes never given me a reason to not trust him before and he was honest about it but what are your Thoughts?