What could cause chronic lower back stiffness symptoms in an 18 year old female?

Many things. Including an old injury, posture problems, not enough exercise, or to much of the same exercise causing an over use injury. Even vitamin d deficiency has been linked to lower back pain.
Growing pains. Often due to rapid bone growth that causes stretching of ligaments attached to bones. If persists more than 6 weeks or if you have numbing or tingling in your legs would consult your physician.

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