My back feels frozen and unable to move when I lie down on my back for a long while. And it hurts. What am I doing wrong?

How far along are U in the pregnancy? By right from 2nd trimester u are not suppose to be lying on ur back as it can reduce the blood supply to the baby due to the wombs weight on the blood vessels. To accommodate the increasing belly infront, ur spine will change its position slightly to arch more. So whenever lie or sit down. Make sure u are well support w pillows to always ensure there is something in full contact w ur back to lend it suppor