I have the sharp pain on my lower back, i cannot sit up straight or walk proper. its very bad. what could it be?

Spondylitis: At your age, provided you did not injure the back, ankylosing spondylitis is the most likely cause of these symptoms. There are a few rarer possibilities. Check with your rheumatologist for an answer and treatment.

See details: Low back pain has so many possible causes that it is impossible to give you a specific answer without examining you and having access to x rays and/or an MRI. See a doctor and have the back pain properly evaluated.