Backache

Why does my back ache so much

Your body is making adjustments for the new entrant. The joints in your pelvis are beginning to loosen up to allow for a smooth delivery. Along with your huge stomach, your body is bound to be slightly off-balance and result in strained back muscles and pain. The correct posture and sensible weight gain would go a long way in minimizing the pain. A general rule is to avoid high heels.

Will carrying my 3-year old put further strain on my back

Yes, most definitely. Try to encourage your child to walk by herself. But do not blame your inability to carry her on the unborn child, as it would lead to sibling rivalry. You could explain that you have a bad back. For times when she insists on being carried, learn the correct way to lift a load.