Fwb almost says he sees black spots after sex with me

I've been seeing a guy for quite a while now off and on and every time we have sex or I give him head he says he is seeing black spots and needs to sit down or says he feels like fainting. He told me he has only ever had that happen with me. At first it was kind of an ego boost but now it is just getting annoying. Am I doing something that is causing this to happen? The last time we had sex after he pulled out if I hadn't grabbed his arm he would have fallen down. He went to his doctor and had a check up done and some tests done on his heart and everything came back normal so that isn't the problem. Has anyone else had this happen to them before?