Cold sores, also known as fever blisters, are a common infection caused by viruses. They are tiny, red, fluid-filled blisters on and near the lips or any other spots on the face. In exceptional cases, you may experience these pesky blisters on the nose, fingers or inside the mouth. Cold sores erupt in patches, where they are clumped together. These blisters may take two or more weeks before getting healed.
Furthermore, herpes simplex, a common virus, is responsible for the emergence of cold sores on the face or other parts of the body. This virus easily spread from one person to another, primarily through close contact like kissing. In addition to that, cold sores are very contagious even for those that are yet to erupt on the face. 
No doubt, stress can interfere with your emotional state. But stress can also bring down your body’s level of immunity, which gives the cold sore in the dormant state a chance to emerge on your skin surface. Therefore, when you are starting to feel stressed, take a couple of minutes for deep breathing and relaxation. Nowadays, there are several ways to manage stress; common relaxation techniques are meditation, deep breath via the abdomen, regular exercise, and yoga.