| There are many possibilities. Swelling of the vulva may be caused by an allergy to a contraceptive, spermicide, or lubricant; a yeast infection or other kind of vaginitis; a lack of lubrication; or a sexually transmitted infection. A change in soaps or laundry detergent could also be the cause. Any of these can lead to irritation of the vagina and vulva and create tiny abrasions that would sting if they came in contact with urine.
Having sex can introduce bacteria into the vagina and urethra, causing urinary tract infections (UTIs). That could create a burning sensation during urination. Some women who use spermicide regularly, with the diaphragm for instance, are prone to having UTIs.
Very sensitive nerve endings in the vulva can lead to a condition called vulvodynia. It causes chronic and potentially severe pain for many women. The pain is often made worse by any kind of physical contact — sex, tampons, or tight clothing. Vulvodynia can sometimes be treated with various prescription medicines.
What to do? Wait a few days to allow the vulva to heal, and then try switching the type of condom, lubrication, spermicide, soap, or detergent you are using and see if the reaction still occurs. If that doesn't help, visit your clinician and discuss your sexual history. The two of you can decide if a physical exam and/or testing for UTIs or sexually transmitted infections might be helpful.
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