Swollen labia after regular, non-rough sex. why?

Allergy, STD, cyst ?: There could be a variety of possible explanations and you should review all potential products as well as any personal history of allergies. It could be a sign of an std and appropriate safeguards should be taken. It could also be a cyst albeit not likely.

Condoms?: If you are using condoms you might have a latex allergy. Part of the orgasmic process is engorgement of the tissue in the genital area and could be normal.