Suddenly don’t trust my partner

I (33M) am suddenly not trusting my partner (37M) when we aren’t together. I went shopping with my friends tonight and could help but wonder what he’s up too. This is new for me, I have never felt this way before but suddenly I do. Like my mind is telling me he’s cheating, and I don’t think he is but I can’t shake it.

We live together and spend a lot of time together. He hasn’t done anything for me to feel this way. This is my longest relationship and first partner I’ve lived with. We’ve been together 6 months.

How do I shake this feeling? I fear it will ruin my relationship…

**TL;DR;: suddenly don’t trust my partner without any idea why, how do I fix it?