2 1/2year old wakes up screaming with a fever of 38.5c and saying head hurts 2 nights in a row.during the day she's happy, active and fine. ?

Look closely: Headache can accompany fever so the issue is intermittent fever. There are many possibilities. Most likely is a viral illness which would end in a few days. Another possibility is a urinary tract infection. That could be checked by getting a clean catch urine. If fever persists for more than 5 days even if child is fine during the day she should be seen by md to check for other problems.

High fever: Can cause headache. Watch the child for few more days. If headache persist, consult with your doctor.