why did pressure build up when u blew air in to the test tube filled with water?

Answer 1

the same way pressure builds up when you blow into a test tube that doesn't have water in it.
Water doesn't compress so you are just putting more molecules that are found in air closer together when you blow air into the test tube. Molecules repulse each other, so a pressure builds up.

Answer 2

Because water is not compressible, and air is.
Because you are trying to fit more mass into the same volume.