Why is part of my wall shiny after cleaning it?
You should get some extra paint keep on hand anyway while the brand is current and the color mix is valid. Since the home is new, you can simply roll over the washed area and blend out in a radial pattern, working to a dry roll. Your shiny spot should all but disappear. 

If the color isn't an identical match, paint the entire wall to the corners without bothering to mask or edge to the base trim or ceiling. It'll be quick and the edging shouldn't show much at all. 

If you're new to painting, do a little reading and dive in. It's a great skill for any homeowner to possess.