Not wanting sex at all (first trimester)

I feel so bad for my fiancé but I haven't been in the mood for sex. I get irritated at him even asking for it. I don't know what to do. I know he's trying to be understanding but I can tell it's getting to him. Everyone says in the first trimester you're supposed to want sex all the time but for me it's opposite. I don't know what to do 😩