Is it possible to get growing pains in just one leg? [just started menstruation when 14.5 years old, so they're definitely still growing] or could it be a sign of something more serious?

Low odds. Growing pains commonly affect children between 4 and 8 years of age (ranging from 3 to 13), are usually bilateral over the anterior thigh or calf or behind the knee. I would advise her be examined by her PCP.

This could be serious or trivial; deserves a comprehensive history and physical. Insufficient information to differentiate. Rather than worry, please see your pediatrician or primary care provider.

"Growing Pains" are a diagnosis of exclusion, meaning you've excluded any other possibility of cause for the pain. You need to start with your pediatrician. Through your growth spurt, are your legs equal in length? Have you developed a scoliosis? Are you an athlete that might have overuse symptoms or some other type of injury. You need to be evaluated for us to give you a good answer...