A p-trap is connected to a drain pipe in a Plumbing system by being installed underneath the sink or fixture. The p-trap is shaped like the letter "P" and traps water to prevent sewer gases from entering the building. The curved shape of the p-trap holds water in the bend, creating a barrier that blocks odors from coming back up the drain pipe.
You usually have to unmount it and get at the bottom.
The shape of the drain. A P trap drains out the wall. It looks like a P laid on it's side. An S trap drains in the floor. Again if you laid the S on it's side.
Probably rust and minerals that came loose when you turned the water off and plugged the screen in the spout. Unscrew the screen the the end of the spout and see if that doesn't fix it.