Right side is
冬 'winter', supplying the sound — dōng drifting to téng with regular d/t alternation. Same phonetic core in
终 end and
咚 boom. A wintry image fits the dull, cold ache of bodily pain — the throb of
头疼 a headache, the chill of
牙疼 a toothache, the long ache of caring for someone you
疼.