Two conditions, one confusing patch
Post-inflammatory pigmentation (PIH) is the skin's response to injury. A pimple, a rash, a burn, a bout of eczema, any of these can trigger localised melanin overproduction as the skin tries to repair itself. The result is a flat, darkened mark that sits exactly where the original wound was. Melasma is different. It is a chronic pigmentation disorder driven by a combination of hormonal activity, UV exposure, and genetic susceptibility. It appears in symmetrical patches, both cheeks, the forehead, the upper lip, and it tends to be diffuse rather than spot-specific.
