A cold sore forms in stages. Early symptoms include itching and tingling on or around the lips before a blister appears, and red or swollen skin. Clusters of fluid-filled blisters similar to tiny pimples develop one or two days later. In the days that follow, the blisters break open and ooze. A scab forms over the cold sore after about a week and falls off in about 10 to 14 days.