What do you think is the reason

So me and my boyfriend been together for a year & four months. We usually are sexually active 3 to 5 times a week. We don't see each other much. But when we do we're usually all over each other. But lately he barely wants to do anything. And if we have sex he never lasts long so I'm not sure if that's why we don't have sex as much as we used to.....i know he gets tired from work. I don't think he would ever cheat on me. But this sucks....it's like i want him and he don't want me. I'm dying for some lovin and i can't get any of it.......he said he wants to have fun in other ways, & i know that we can..but I'm not understanding why he isn't engaging in sex with me as much. It's bumming me out. I'm starting to question if there's something wrong with me.. he knows i always want it but i don't know