Why does my stomach become sore after sleeping on my back and sides? i am 16 weeks pregnant.

Growing changes anat: You have round ligaments left and right sides of uterus that attach to pelvic side wall. They stretch slowly accommodating the fast growing uterus. So as long as changing position helps without obvious strong menstrual cramps vaginal bleeding heavy discharge the ligaments will eventually accommodate and soften. No back sleeping after 20 weeks. Update the ultrasound to check the cervix length.

Pregnancy pains: You can have "stomach" pain from muscle stretching, full bladder, etc. You should see your doctor and tell them.