What Causes Sweaty Hands, Heart Palpitations And Fainting During Pregnancy?

I am pregnant and I keep having these things where my hands get really sweaty, it is hard for me to hear (sounds like I m in a bucket), my heart feels like it is beating fast and hard, everything goes black and if I don t sit down I pass out. It happens a lot and even some when I have been in a store. It is not a certain time of day or action that makes it happen.