Stress can indeed cause a rise in the white cell count, or it may be that you recently had a viral illness such as a cold.
You are correct that some blood cancers (leukemia) can cause an increase in the WCC, but if this was the case, you would be showing other symptoms too, such as swelling of the lymph nodes in your neck, armpits and groin, as well as an enlarged spleen. This finding is incidental, and most likely represents nothing serious. Your WCC isn't alarmingly high, which is what one would expect to see when the bone marrow is over-producing white cells at a rate related to leukemia.
If your next count is high, they may do more retesting, such as flow cytometry (this can diagnose exactly what white cell is elevated, and if it contains abnormal DNA seen in cancers).
As you can see here, there are many harmless reasons for an elevated WCC: