I see that you live in Sycamore Ridge and that your assessment decreased 2012 to 2013. Is it the 2014 assessment that you are concerned about?
Please contact me to get an analysis of the that areas recent activity.
Metro Center KW
All the best to you.
Although you can try doing it yourself--Fairfax County has property sale information online--it'd be a lot easier to get a Realtor to run some comps for you. There shouldn't be any charge for that. [Not a solicitation, but if you send me your address, I can try finding comps for you.]
Hope that helps.