Yes, txsilver is right about asking what medications you are currently on for your RA (if that is what you have, this is the arthritis forum). Many of the most recent arthritis medications can surpress the immune system, thus a low white count.
Medications most likely to do this are the TNF inhibitors and Rituxan.
Hopefully it was a lab snafu and your retest will be fine.
I am glad your GP called and that you are going back in. Are you certain it was the flu? What medications are you taking for your RA?