An S-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 and design. An S-trap has a curved pipe that forms an "S" shape, while a P-trap has a curved pipe that forms a "P" shape. P-traps are more commonly used today because they are less likely to siphon dry and allow sewer gases to enter the building.
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.