I have been experiencing pain in my lower abdomen every time I engage those muscles since I had a baby five months ago. What could it be? OB stumped.

Many things. A full evaluation and examination will be needed to determine the cause. An abdominal wall hernia is one possibility. Diastasis recti is another. An in person evaluation is the next step.
Ab muscle separation. You could be experiencing separation of the abdominal muscles called Diastasis Recti. During pregnancy the abdominal muscles stretch, to accommodate pregnancy. After child birth, the muscles oftentimes don't return to previous form. Further evaluation w/ ultrasound may help diagnosis, & physical therapy may treat it when diagnosed. Worse cases may cause a hernia which may need surgery.