Work backward from the timeline. The swelling appears immediately after a forceps delivery, which points to trauma rather than infection or a developmental lesion.
Forceps blades can tear small vessels in the paravaginal connective tissue while the overlying mucosa stays intact. The escaping blood is trapped in a confined plane, so it forms a tense, exquisitely tender, fluctuant lump. Because the mass lies adjacent to the lower urinary tract, its pressure on the urethra explains the difficulty in passing urine.
Contrast the distractors: a urethral diverticulum is chronic with post-micturition dribbling; a cervical tear bleeds briskly from the canal rather than producing a vulval lump; an abscess needs days and brings fever and redness. None match an instant, painful, fluctuant post-forceps swelling.
$Acute + painful + fluctuant + post\text{-}instrumental \Rightarrow$ a blood collection.
\[\boxed{\text{Vulvovaginal (puerperal) hematoma}}\]