Welcome to Healthcare Magic forum.
Considering your description your daughter seems to have a petechial rash
or a ecchymoses (bruise) . The cause for them is bleeding from the small vessels under the skin either due to trauma, allergy(pollen or plant , insect bites
, paints or chemicals or harsh sun rays.etc)or an infection or blood disorfer.
In her case it was sudden, acute so most likely to be either traumatic or allergic.
The fact that its localised to the back of her arm and leg, favours more towards a traumatic cause.
Many a times, children do not heed to cuts and bruises while playing only to find them painful during rest or sleeping.
The best way to confirm is (as you know), a physical examination by her pediatrician
who can come to the exact diagnosis and appropriate management.He may order a blood test
to rule out any bleeding disorders or vitamin deficiencies which can frequently cause such bleeding even without any trauma or minimal stress.
The complications which can arise if neglected are, spread of the rash , oral lesions if allergy or infection and internal bleeding
if blood disorder or severe trauma.
There are chances that the lesions may not spread and gradually change color to brown , green and fade away
Assuming, she has no other complaints like fever, bleeding through any other site (blood in stools, urine or cough), no swelling , breathing difficulty,no major pains, I would say that the situation doesn't seem to be an emergency and you may wait and watch for it's resolution.
Meanwhile you may try the following for relief.
1.Apply ice or wash the area with cold water .Apply ice for about 15 minutes at a time, and always put something like a towel or wash cloth between the ice and the skin.
2. For pain, you may give her analgesics as per your doctors prescription Usually acetaminophen
is preferred over ibuprofen
which can prolong bleeding.Pain and tenderness should get better within a few days and the bruise should go away within a couple of weeks.
3.if its itching or burning local application of calamine containing lotions may provide soothing relief.
4 If the lesions increase in number or she develops any of the above mentioned symptoms please seek prompt consultation with her doctor or any urgent care facility.
Hope that answers your query.
Wishing her good health.