Why I see red rectangle along my screen?

If you mean a red rectangle all around your screen, this sounds like an android developer's option. It indicates that the main thread (the one that processes your keyboard and touch inputs) is busy.

Developers use this to make sure that they are not monopolizing the main thread, which is a BAD THING TO DO. A virus or piece of bad software would not care about doind BAD THINGS, of course.

Back up your tab, boot it in safe mode, delete the apps you've installed, and reinstall them one at a time, slowly, until you find one that misbehaves.

You can do a factory reset if that doesn't work. Be sure to back things up first, though.