Phimosis or Tight Foreskin: Pain immediately after oral sex could be due to forced retraction of a tight foreskin, leading to minor tears or irritation, especially if the individual has phimosis.

Friction or Irritation: The mechanical action during oral sex can cause friction, leading to superficial pain, especially if there was insufficient lubrication or if the foreskin was pulled back too vigorously.
Allergic Reaction or Contact Dermatitis: An allergic reaction to a sexual partner's saliva, spermicide, or other substances could cause pain and irritation on the glans.
Herpes Simplex Virus (HSV) Infection: Although the pain from HSV is typically more severe and accompanied by vesicles, an initial outbreak could present with sharp, knife-like pain, especially if the individual has never had a previous outbreak.

Gonorrhea or Chlamydia: These sexually transmitted infections (STIs) can cause urethritis, leading to pain during or after sexual activity. While they might not typically cause a sharp, superficial pain, missing these diagnoses could lead to serious complications if left untreated.
Syphilis: Primary syphilis can present with a painless chancre, but secondary syphilis can cause a variety of symptoms, including pain during sexual activity due to mucocutaneous lesions.

Fixed Drug Eruption: A rare condition where certain medications cause skin lesions that recur at the same site each time the drug is taken. If the lesion is on the glans, sexual activity could exacerbate the pain.
Behçet's Disease: A form of vasculitis that can cause genital ulcers and pain. It's rare and would typically be accompanied by other systemic symptoms.
Squamous Cell Carcinoma or Other Malignancies: Although extremely rare, any persistent or severe pain should prompt an evaluation for malignancy, especially in individuals with risk factors such as HPV infection or a history of smoking.