Why does under my right vagina lip appear swollen and red and hurts a little?

Blockage likely. You likely have what is called a "bartholin gland cyst." the bartholin glands produce mucus to help defend the vulva against infection, but sometimes can get obsturcted in a similar manner to a pimple. While this is a likely explaination, you should see a physican to make sure it isn't something more serious. The best initial treatment would be a "sitz bath" (warm water 2-3x day).
Possible Infection. Either an infection of the right bartholin gland or a hair follicle infection. See your gyn doctor to be evaluated.

Related Questions

How do you know you have a vaginal cyst? Or an ovarian one? Why would my right half of the vagina under the lip be swollen and red and hurts a little

Bartholin cyst ? There are lubricating glands inside the vaginal opening. These can become blocked, and swell up, causing some pain also. Please see your doctor, and have a pelvic exam to determine the cause, and get some treatment. Read more...