Laying down while pregnant? Bad or??

Ok so yes the question is written weird haha But the real question is... I have heard a lot of things about laying on your back while pregnant. I am 27 weeks pregnant and I lay on my sides due to all I’ve read about laying on your back and let me tell you all that my back kills :( I once in a while lay on my back to relive the discomfort for a mins or two ( & I end up falling asleep) and I wake up freaking out and immediately turn on my sides. Nothing hurts while I’m on my back and I don’t feel dizzy like most post say. So here’s the question finally: “Can anyone tell me the exact dangers and why we can’t lay on our backs while pregnant?? Also during intercourse I am sometimes on my back is that harmful as well?!!