I'm 23.4 weeks pregnant what is causing my really bad lower back pains?

Loose ligaments. There can be a number of causes for back pain in pregnancy. One of the most common causes in my opinion is the fact that as the baby grows, a womens pelvic ligaments will start to stretch and relax in anticipation of the delivery. This can cause back pain. A retroverted uterus can also be a source of back pain. The good news is that the back pain will usually go away after delivery. Talk toyour md.
Hormones & Changes. Back pain in pregnancy is extremely common. Hormonal changes cause loosening of ligaments and lead to some instability which can cause strain on muscles the body is not used to using. Increased weight concentrated in the abdomen causes weakness in abdominal muscles, as well as a shift in center of gravity leading to a change in posture and increased pressure on the back.