At this stage the added pressure on your back might make you feel uncomfortable. If your lower back has been aching, it’s because of your growing uterus, and the hormonal changes that loosen your joints and ligaments.
Plus, the extra weight you're carrying means more work for your muscles and increased stress on your joints, which is why you may feel worse at the end of the day. Walking, standing, or sitting for long periods, as well as bending and lifting, can put a strain on your back.
Avoid sleeping on your back completely from now on and use a body pillow for support. The pillow will also help you stay on your side.