What Causes Itchiness In The Waist And Armpit?

My waistline get's VERY itchy with some darkened areas (almost like bruising) it's been random for the past few years. Sometimes it's gone for months then comes back for a few days/weeks.
this time I even had it itchy in the front side of my armpits. There isn't a real noticeable rash, just looks like darkened dryness. and is VERY VERY itchy. The hot water of the shower makes the itching worse. Any ideas?