theres time I don't lay flat though I'm actually laying on my side when it happens
I am 35 weeks an 2 day an that has been happening to me since week 25 just put a extra pillow under you head an it will go away but you can't lay totally flat because of the major vain that runs from the top of your body to the lower part that supplies all the blood the weight of the baby cuts it off when you lay all the way flat I hope that helps
Are you lying on your back? Baby can cut if circulation.