Ingrown hairs occur when hair curls back or grows sideways into the skin instead of rising up from it. This often causes red, itchy bumps that resemble pimples. In some cases, ingrown hairs can become infected, resulting in painful pus-filled lesions.
Causes

Shaving, waxing, or plucking hair too closely
Wearing tight clothing that causes friction
Naturally curly or coarse hair
Symptoms

Small, round bumps that may contain pus
Redness and inflammation
Itching or discomfort in the affected area
Treatment
Apply a warm compress to reduce swelling

Use exfoliating cleansers to remove dead skin
Avoid shaving until the area heals
In severe cases, consult a dermatologist
Source: American Academy of Dermatology