am I doing something wrong?

Ive been seeing this guy for a while and it started as just a sex thing however it had become more "relationshipy." We clearly have a lot of feelings for each other but haven't talked about it. The sex is so good it's insane however, the last two times we had sex he hasn't finished or has struggled to get it up and keep it up. I care about him and I want him to feel good. Do you think he's nervous? Both times he said he was just tired but idk. What causes this, is it me???