Sex issues

To start off, I don’t have much experience in sex. I’m currently engaged, I’m 22 years old and my fiancé is 23. There has been a few times when my fiancé sudden turns off, even while we’re having sex. It has bent a few times which turns him off because it really hurts him. I understand that. Today, he was the one who wanted to have sex but he suddenly turned off. I told him we can stop if he’s tired because he did just got off of work. Instead, he wanted to take a pause and try it again but he turned off again. I’d ask him if he’s okay or if everything is okay or if I’m not doing good but he assures me that he is perfectly okay and that he doesn’t even know why it happens. He has mentioned that it’s like it has a mind of its own, it’ll stop working when it wants to and also when we have a sex the previous day or 2, it does tire him out. This is perhaps the fifth time but it doesn’t happen frequently. Though, it does concern me because I don’t know what that means or am I really just not good enough sometimes?

Anh advice/tips or idea what this could mean?

Should I be really concerned?