@orangey03 – For a child and especially for her parents, something as strange as purple spots under the skin can be frightening. When I saw a petechial rash on my daughter's neck, I grew very worried. She pointed it out to me with tears, and I called the doctor right away.
She had bronchitis at the time, so she had been coughing hard nonstop for three days. The doctor said that this was the cause of the rash.
Since she was already taking antibiotics to resolve the cause of the coughing, there was nothing to do but wait. If her rash had appeared and she hadn't been coughing so forcefully, she would have needed more tests.
The doctor seemed pretty certain of what it was, and he turned out to be right. The rash went away on its own in a week.