This eruption is more common in warm climates or on sunny holidays. It is caused by a yeast growing on the skin in warm conditions and causing discolouration of the skin (sometimes paler, sometimes darker).
The condition is sometimes mis-diagnosed as eczema
but responds to specific treatments directed against the yeast.
As a common condition yet often poorly recognised, many patients continue to have patches of abnormal skin for many years when the treatment is in fact very simple. Consider an expert opinion with a Dermatology Consultant if you are worried. You can use 'consult the specialist'