What does it mean when experience lower abdominal pain that also includes back pain during second trimester when pregnant?

It's the way you are wired. When the sensory nerves in your uterus fire, your brain interprets the signal as coming from your pelvis and back. It's a pain in your belly that "radiates" to your back. It's helpful to distinguish it from actual back pain that arises from carrying the extra weight of pregnancy. If the radiating belly pain becomes severe, see a doctor right away.