Is she on any medication?My 11 yr old daughter is bipolar and when she was on depakote here white blood cell count went down.Also antipsychotic meds will do the same. When she got off she was fine.
Get her tested again. It could be something called Pseudoleukopenia, which happens at the onset of infection.
There are a number of things that can cause this. You may want to check out the following link from the Mayo Clinic on the various causes.
http://www.mayoclinic.com/health/low-white-blood-cell-count/MY00162/DSECTION=causes
it usually means she is or has been sick, if you get it done again probably if the infection has gone it will be normal ,it it a good idea to have the test done again. Was she sick at the time or had recently been sick , Shje is very likely okay but have it tested again for your own peace of mind and ask the doctor questions about it ...good luck