what will cause vagina lips to swell?

Answer 1

There are so many things that can cause this that it is best to see your gynecologist. For example, some causes are:
jeans worn too tight (or other type of pants)
allergy to latex condoms
allergy to contraceptive creams
allergy to dye in underwear
yeast infection
rough sex
some STDs
etc.

Since you didn't give much information or list any other symptoms, it is hard to narrow it down any further. Please see your gynecologist for a formal diagnosis and any treatment that is necessary.