5wks pregnant, lower abdomen feels a little like having done sit-ups when I bend over. Normal?

Yes. It is normal to feel mildly achy in the center of your low abdomen in early pregnancy. As the uterus starts to grow and stretch it can cause light cramping. Worsening pain, sharp pains focused on one side, and bleeding that early in pregnancy can sometimes be a sign of a more serious problem like a threatened miscarriage or ectopic pregnancy.
Stretched. Hernias are common during gestation. Care should be taken to avoid strenuous lifting to avoid an abdominal strain and in rare cases herniation. If spotting occurs. You should immediately seek medical care as the cervix may be involved.