Relationship trouble

I have a feeling that my fiancé is doing something behind my back. Not sure exactly what if it’s cheating or talking to someone but I get this vibe that something is happening. He’s always quiet says we don’t always have to talk but he’s overly quiet. He’s always on his phone when I leave the room, he gets texts late at night and I ask him who it was and he says he doesn’t know. I left the room to let my dog outside and his alarm went off he shut it off faster than usual. He hasn’t wanted sex in a few days. When we did the other day he said it was amazing. Our relationship has been kind of rocky for a few now. Not because I think something is going on I’m waiting to see if something surfaces from it. He gets mad when I ask questions. Most men I’ve dated would rather ask a question then to assume, but he’s the opposite. I know nobody here can confirm or deny what’s going on, I wanted to vent a little get it off my chest. It’s really bothering me. Then when I ask him about things he will say nothing is going on. Guess I have to believe him until I have real facts about the situation. Maybe it’s nothing and I’m overthinking it. Not sure what to believe. Oh yeah I forgot to add he seems to think I’m the cheater. Smh. I’m not a cheater. I’m actually a really loyal person.

Also his grandma just came in to make sure he’s up for work. Like if he wants to marry me I feel like that is my job to help lmao. I guess not. I don’t intend to actually marry this guy he is super immature for me, so I’m not really sure what I’m doing with him atm. Smh and yes I’ve expressed to him how a relationship or being married works but nothing ever happens so at this point, there’s no more talking I can do to even attempt to save or fix anything.