What Causes Pain And Swelling In The Calf?

My daughter is 16, 5'3" 127lbs. She runs track and field for USATF. She has been struggling with pain in her left calf. She has been going to a sports trainer for over 4 months 3 times a week. She is in great shape and has done all kinds of exercies to strengthen her calf and surrounding mucles. Her pain is not getting better. The swelling is constistant daily.