Passing out after sex

This has happened a few times but more frequently since I got pregnant. I'm currently 6 months. But after my husband and I have sex, I'll go to the bathroom and after a while I'll start to feel hot, dizzy and nauseous. Then I'll lose my vision and my ears will start ringing. The time before this one, I literally had to crawl off the toilet before I fell off of it. Now I'm currently laying on the floor on my back because I felt it happening again. Does anyone know what this could be? Doctor's appointment this week and I'm definitely asking her but that's not for a few days.