Does appearing after sex mean it came from that encounter?
Not necessarily. Friction can make an existing skin problem noticeable, and infections have different times before symptoms appear. The day a sore appears cannot reliably identify who passed on an infection or prove infidelity.
What could cause it?
Friction or irritation
Rubbing, dryness, shaving or a reaction to a product can injure sensitive skin.
Infections
Herpes, syphilis and other infections can cause genital ulcers. Different conditions can look alike, and more than one cause can be present.
What should you do now?
Avoid sexual contact involving the affected area until assessed. Do not pick, squeeze or apply strong antiseptics, random creams or leftover antibiotics. Arrange an assessment while the sore is visible; a swab or blood test may be appropriate.
What will the clinician ask?
Be ready to explain when it appeared, whether it hurts, whether there are blisters or other symptoms, and any recent sexual contact. Mention pregnancy, medicines and anything already applied to the skin.
Common questions
Can a painless sore be an STI?
Yes. Some STI-related sores are painless. Lack of pain does not remove the need for assessment.
Can I tell the cause from a photo?
A photo may help a clinician, but examination and testing may still be needed. Appearance alone cannot confirm every cause.