Separation anxiety is a normal stage for kids to go through.  It starts around 6 months and usually tapers off around 2 years old.  During these months a baby is first gaining the cognitive recognition that you still exist when you are not there, which means baby can now miss you when you are not there.  The problem often intensifies because at the same time baby realizes that her primary source of food and comfort can leave her, she is also testing the ways in which she is an individual.  That's scary!  A lot of separation anxiety is about finding that fine line between growing more independent and at some level still knowing she is fully dependent on you for survival.