After stress and a flu my skin became very dry, my scalp is dry, less hair. but no rush or dandruff. is it dry seborrhoeic dermatitis? how to treat?

Stress / cortisone: Whenever the body is stressed (whether it's a physical stress like the flu, or it's a psychological stress) cortisol is released. Stress hormones like cortisol can affect the body's hair cycles (in the most noticeable form this is called telogen effluvium). Unless you can cure stress, there's nothing you can do to prevent it or treat it! Nothing to worry about though.