Do I have a skin disease on my waistline where the belt buckle is? There's some rash there.

Nickel sensitivity. The most common reason to have a rash on you waist near your belt buckle is due to nickel sensitivity. Nickel is a metal which is commonly used to coat other metal items, such as belt buckles, snaps, jewelry, etc. Ir you become sensitive to this, then any contact can lead to a rash. Use 1% Hydrocortisone cream if itchy and avoid contact with the metal. See your doctor if it persists.