头昏 is a noun meaning 'dizziness', but in Chinese it is very commonly used as a predicate (like a stative verb) to describe feeling dizzy or giddy. You'll often hear it in patterns like 我头昏 ('I feel dizzy') or 有点头昏 ('a bit dizzy'). It can also be modified by degree adverbs: 很头昏 ('very dizzy'). Despite the noun POS label, its most frequent use is adjectival-predicate.