Try this 10 min sequence and see if your low back muscles can lengthen.
Grippy low back muscles are very likely to contribute to the discomfort in your low back. If the muscles are shortened on one side only, they pull on your pelvis laterally which can lead to compensation patterns in the way you walk and sit.
If the muscles are short on both sides, they can pull your pelvis into an anterior tilt which will make it harder for your core to support your low back and ensure proper breathing mechanics.
If you are pregnant, a tilted pelvis affects the way your uterus is positioned inside your body which in turn affects the space that your baby has to navigate to find the most comfortable position for birth.
Try noticing your habitual ways of tilting the pelvis when you sit (as in crossing your legs), stand (as in putting your weight always on the same leg) and walk (maybe carrying something or somebody on one hip) -- the way we use our body shapes our body. Every time your notice a tilt in the pelvis, gently correct it: uncross your legs, distribute your weight evenly between two legs, use your arms when you carry rather than propping your load up on one hip. Do this release often, gradually work on changing your habits and see what a difference it can make in how your low back feels. P.S. If you find it challenging to tuck and untuck your pelvis, practice with this video first before progressing to the Hip Dip.