I often getting pain on my left lower rib.i am not sure when it is coming.it is coming suddenly like a sharp pain.

Pleurisy?: This may reflect pleurisy, or more likely, tietzies syndrome, chostochondritis, an inflammation of the cartilage of the rib. If you can put your finger right on the spot which hurts, it is more likely to be tietzie's syndrome. If not, you need to get a chest x ray.

Unclear: You are describing pain in the left lower rib cage are female and thirty. Start with writing down each time you get it. Note the time, when you last ate, what you were doing right before and what if anything made it go away. If you are getting these pains once a week then take this information to your doctor and he/she can start to help you. Hopefully it will be self limiting.