What Causes Lightheadedness, Nausea And Cold Sweats?

Hi I am almost 30 weeks pregnant. So I was having a bowel movement the other day and out of nowhere I got super lightheaded nauseous and I ended up with really bad cold sweats. I got really scared because I was alone and I didnt want to pass out and hit my head or anything like that. So I flushed the toilet and just sat in my bathroom until I could manage to move. So do you know what is wrong with me?