Boyfriend of 4yrs told me after sex

Hi, just wanting to know if anyone else’s partner has been through this. Last night after sex my partner of 4yrs went pretty distant with me which was unusual. I asked what was wrong and he said just sometimes after sex he feels down. Like he’s on a come down or something. He then went on to say he’s not always felt like that after every time but pretty much from the first night we ever had sex he’s been getting it on and off. He said it doesn’t last very long afterwards like but it’s made me feel some type of way that I can’t fully explain. It upsets me that he feels down afterwards because I don’t like the thought of him feeling down about anything in general I want to help but he doesn’t know how I can help him. I’ve looked it up and it’s common but can be caused by so many different things. I’ve never heard of it before so just wondered if anyone else is in the same boat or their partner has felt like that before and what helped? TIA