anxiety in children

just after any input or experience with children who have anxiety disorders? i am growing more and more concerned that my DD1 who is 5 may have some anxiety issues that she isnt growing out of. mainly, tantrums with no seemingly obvious cause.. (usually to do with seperation) not able to verbalise the reason why she is crying. perfectionsit personality.. she can get very distressed when she cant do something perfectly or becomes upset when things dont happen in the way that she has preconcieved that they are going to occur. she has been waking up with headaches, went through a period of biting her lip so much that it became a sore, takes a very long time to calm down after a crying/upset episode. she worries about things that she doesnt need to worry about. and a few other things, but her behaviour has been quite difficult in general lately. she has also started lying, and being defiant. i really feel that she is a good kid, knows right from wrong but of late she is really testing the boundaries. or even, sometimes i think she tries not to become clingy etc but it just happens and she doesnt know why or how to settle herself down.

her tantrums/clingy/crying episodes are very erratic. for instance, she cried when playing pass the parcel because everyone was looking at her, yet she happily played it at a party the very next week. she gets overwhelmed when we enter a large group of people ( mothers group family get togethers) even though she has known them for years yet the next group meeting she will be fine. its hard to know what is triggering her off.

i want to know what anxiety disorders exist with young children. what can i do to get her diagnosed (if anything even can be diagnosed), how to help in situations that she is clingy and crying etc. i dont really know what to do and i find myself getting angry with her which i am sure doenst help.

thanks.