Currently 18 weeks pregnant and have really bad sciatica or pelvic girdle pain. It's at it's worst when I carry my toddler and at night. What to do?

Lots of Reasons. When pregnant, there are so many things going on. Your hormones are very different causing your ligaments to stretch quite a bit especially in the pelvis resulting in pain. Also the baby's position can place stress on your spine resulting in bulging discs or even herniated disks causing the sciatica pain. Recommend seeing a pregnancy specialized therapist that can guide you through this.
Dont carry your todl. As above. Give your back a rest.