Unlike people with dry skin, the
skin in people with seborrhoeic dermatitis is usually
oily.
In adults seborrhoeic dermatitis usually affects the scalp, eyebrows, ear canals, sides of the nose, and behind the ears. It sometimes affects the armpits, chest and in the groin area.
Stress, fatigue (pregnancy / lactation in your case), weather extremes, oily skin and infrequent shampooing or skin cleaning & severe medical illnesses make it worse.