What are causes of a swollen labia minora?

It depends: The level of "trauma" to the labia minora can be minor or more severe. Sex that is too long, too rough, too dry etc can cause severe swelling. It is also possible that you have developed an infection, std or some other problem. If the swelling does not resolve, you need to be examined, call your doctor!

Excitement for one: Excitement, trauma, allergy and infection are all common causes.