Hi, welcome to HCM.
in last trimester of pregnancy
is not a pathological pain. I5 occurs due to change in center of gravity of body.
As there is a foetus in abdominal cavity mother has to lean backward to maintain body posture . It alters normal spinal angle which is main cause for pain. It will get resolved after delivery.
You need bed rest
till delivery. Avoid medications as much as possible.
Hope I have answered your query.