Laying on your back...

I learned yesterday why they tell you not to lay on your back while pregnant. Im almost 23 weeks. Well yesterday i went in for an ultrasound (where the woman takes all the measurements of your baby). About 20 minutes into the ultrasound, i got extremely light headed,started sweating and felt like i was gonna puke! She made me roll over to my left side, and luckily those feelings went away. Just a little heads up if anyone wondered why they tell you not to lay on your back!