Part of my nostril become red and it's hurt when it was touched. what happen?

Infection: It is likely a local infection. Should improve with warm compresses (hot water on a wash rag) to area a few times a day. If spreads or not improving in 2-3 days, see dr.

Infection?: It sounds like a superficial infection. See your physician to determine if you need topical or oral antibiotics, or possibly incision or evacuation.

?pimple: A pimple on the very sensitive part of the external nostril (or blackhead or cyst) can be excrutiatingly painful. Such bumps are likely to occur with times of frequent nose wiping, but can occur any time, especially in those prone to cystic acne such as teenagers. Try not to touch it pop it or squeeze it, this too will pass. If you absolutely have to apply warm compresses to move it along.

Cellulitis?: Sounds like it could be a skin infection called cellulitis. Plucking a nose hair? If the redness and pain spreads, seek medical attention.

Cellulitis/infection: Painful red skin suggests infection. Go see your doctor for evaluation, diagnosis & treatment (antibiotics). Consider warm compresses temporarily to affected (ice is better for injuries).