What are these stains on my drywall?
Knot stains happen when the sap of real wood transfers through the paint.  Knots are full of sap, and don't fully dry out when the boards are processed.  This can't happen with drywall.

What does commonly transfer through drywall is water, and I would guess that what you have here is a water stain.  It could be a leaky pipe fitting, or it could be condensation from an A/C line if that happens to run through the wall, but I admit that's probably not very common.

Since the house is so new, just call the builder.