Here is a slightly diferent take - perhaps she has a retained Moro Reflex, which can turn into a startle reflex as she gets older. Unfortunately, a retained Moro means a future of being very sensitive, both inside and out, anxiety (separation and eventually test), hypoglycemia, allergies, and trouble fixating on an object with her eyes. My child had this until an OT helped her in 6th grade; I had this until a couple years ago.
My suggestion is to ask your doctor if she still retains the Moro reflex. If the answer is yes, play games that will help her integrate it - peek-a-boo, blowing in her face, putting a blanket over her and drawing it off suddenly. There is an exercise called starfish which will help, but she may be too young to practice it, as it is fairly complicated.
Meanwhile, play with her on the ground, on her tummy and on her back everyday so that she gets her other reflexes integrated too. Not only does she develop physically, but also her brain develops at this point in time, so that she can process sounds and visual stimuli correctly, maintain attention, develop a good memory, etc.
It is an exciting time in her life. If she continues to be anxious - more than others her age - you can ask for an OT evaluation.
Good luck!
K. Johnson, MS Ed