I am a girl and work out regularly, mostly cardio and less weight training. I haven't fallen down anywhere. Even then, my back gives me pain when I stand up. What could be the reason?

Cardio is good.

But in order for body to bear day to day stress it has to be conditioned.
Body weight resistance training or weight training is an awesome way to
strengthen you ligaments and tendons.
Loading a weight on body improves the structural integrity,
Bone density improves, hence they become strong.

Back pain may be a result of variety of reasons like
crappy posture, muscle imbalances, inadequate rest.

Weight training improves posture.
you should look for any imbalances if they exist in your body,
corrective exercises can help overcome any imbalances.

Try including compound exercises in your fitness schedule.
Deadlifts and Squats help to build a strong Posterior Chain muscles.

Hope it helps.
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