Thank you for asking the query.
Since you are pregnant this back pain could be pregnancy related back pain.
Now there are many causes for the pregnancy related back pain. Your muscles, especially in the lower abdomen and back will relax due to the pregnancy hormones and this can cause some changes in the spine. Since you are having pain while lying down and trying to move, it could be related to a disc herniation. - This again secondary to the pregnancy . The herniated disc and compress the nerves causing.
For the pain you can try hot and cold compress / mild non weight bearing exercise like swimming / local pain relief sprays and ointments. Also do not carry heavy weights. If you need to pick something up from the ground, use your legs to squat rather than bend over.
Further testing is need to confirm the cause, but its been done after the pregnancy (The pain might even disappear completely) . However you should watch for the following danger signs and contact your Doctor immediately if you have any of them.
1- Numbness /loss of sensation in the groin of legs.
2 - Lower limb weakness.
3 - Urinary symptoms like unawareness (Frequent urination is common in pregnancy).
Hope this helps. Please feel free to ask follow up questions via this email itself