A cold sore usually passes through several stages: 
Tingling and itching. Many people feel itching, burning or tingling around the lips for a day or so before a small, hard, painful spot appears and blisters erupt.
Blisters. Small fluid-filled blisters typically erupt along the border of your lips. Sometimes they appear around the nose or cheeks or inside the mouth.
Oozing and crusting. The small blisters may merge and then burst, leaving shallow open sores that ooze and crust over.
 Signs and symptoms vary, depending on whether this is your first outbreak or a recurrence. The first time you have a cold sore, symptoms may not start for up to 20 days after you were first exposed to the virus. The sores can last several days, and the blisters can take two to three weeks to heal completely. Recurrences typically appear at the same spot each time and tend to be less severe than the first outbreak. In a first-time outbreak, you also might experience: 
Fever
Painful gums
Sore throat
Headache
Muscle aches
Swollen lymph nodes
 Children under 5 years old may have cold sores inside their mouths and the lesions are commonly mistaken for canker sores. Canker sores involve only the mucous membrane and aren't caused by the herpes simplex virus.