What Causes Pain Between Wrist And Shoulder In A Toddler?

my child is 2 years of age and she was playing in her bedroom when she started crying she has a travel cot which she climbs into to get to toys and i found her sitting in her cot holding her forearm has been having difficulty using her left arm from since yesterday she complains that it hurts her between her wrist and her shoulder and is reluctant to try and use or let you touch or hold it.she has slept through the night and will not hold objects in that hand, please advise as to what i should do?