Contact Dermatitis or Irritation: Given the recent sexual activity, the red spots could be due to friction or an allergic reaction to a product used during the encounter, such as lubricants or condoms. The fact that the individual is concerned about the timing in relation to sexual activity suggests a possible link.

Allergic Reaction: An allergic reaction to something the individual came into contact with, which could be unrelated to the sexual activity but coincidental in timing.
Folliculitis: An inflammation of the hair follicles, which can appear as red spots and could be caused by friction, shaving, or infection.
Heat Rash: Given the physical exertion and potential for increased body temperature during sexual activity, a heat rash is a plausible explanation.

Primary Herpes Simplex Virus (HSV) Infection: Despite being vaccinated against chickenpox (varicella), the individual could still be susceptible to HSV, which can cause painful blisters or red spots. The recent sexual activity increases the risk of transmission.
Syphilis: A sexually transmitted infection that can cause a variety of skin manifestations, including red spots or rashes. Early detection is crucial for effective treatment.
HIV-related Rash: Certain rashes can be associated with the acute phase of HIV infection. Given the recent sexual activity, this is a critical diagnosis not to miss.

Erythema Multiforme: A skin condition characterized by target lesions, which can be triggered by infections (including HSV) or medications. It's less common but should be considered if the rash has a distinctive appearance.
Kawasaki Disease: Although extremely rare in adults, Kawasaki disease can cause a rash, fever, and other systemic symptoms. It's more commonly associated with children but should be considered in the differential diagnosis of any unexplained rash, especially if accompanied by other systemic symptoms.