I fell and got a bump on my forehead.
I bumped my head and got a big lump.
Katakana spelling, often used for emphasis or in casual writing.
Rare kanji form; the reading たんこぶ is not obvious from the kanji.
The origin is uncertain; likely a colloquial formation from たん (possibly related to たんこ 'lump') and こぶ 'lump, bump'.