A U-trap and a P-trap are both types of traps used in Plumbing systems to prevent sewer gases from entering a building. The main difference between them is their shape: a U-trap has a U-shaped bend, while a P-trap has a P-shaped bend. Both traps serve the same purpose, but the shape of the trap can affect how effectively it prevents odors from escaping.
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.