Adolescence can be a very difficult time. You are no longer a child, but you are not an adult yet either. Hormones is the answer. Your brain knows when your body is nearing sexual maturity. So it releases chemicals called hormones.
Different hormones are responsible for different changes.
As you get nearer to puberty, the brain and pituitary gland release hormones that regulate the reproductive organs of both male and female. These hormones stimulate the ovaries of girls to produce another hormone called estrogen, and the testes of boys to produce testosterone and sperm. Arms and legs get longer and internal body organs get larger. Your body is experiencing a major growth surge. This surge lasts about two to three years.