It does sounds like you have too much paint loaded on your brush. A wash should "stain" the paint underneath, but only in the area you apply it.
I wouldn't run out and buy Matte Medium just yet. If you are starting out, try and keep it simple. Have a look at this excellent tutorial on the Reaper site, its called How to Blend, but olliekickflip is really talking about washing/juicing technique.
http://www.reapermini.com/forum/index.php?showtopic=32050&st=0