This morning seemed like any other, but when you made it to the mirror, you noticed a painful little bump near your mouth. It could be a canker sore — or it could be a cold sore, also known as a fever blister, also known as herpes.Although canker sores are caused by some sort of mouth trauma, such as accidentally biting the inside of your cheek, cold sores are caused by the herpes simplex virus, explains Cheryl Iglesia, MD.But it's not always that easy to tell the difference — especially if this is your first time dealing with either. So we asked Dr. Iglesia about what you need to know about both.Canker sores have a distinct look: They're usually round and white, with a red border. They tend to appear (inside the mouth only) after biting or brushing your cheek or gums too hard, says Dr. Iglesia. These aren't related to herpes, but they are associated with other diseases (such as Crohn's and celiac) and with some vitamin deficiencies (such as B-12). Canker sores can be very painful, especially when they're big, but they do generally heal on their own within two weeks. So you usually don't need medical attention for a canker sore unless it's making it hard for you to eat. If it's that bad, your doctor can prescribe a mouth rinse that can speed healing and reduce pain. Over-the-counter numbing medications may also be helpful.On the other hand, the sores caused by herpes look like small, fluid-filled blisters and may appear in clusters. In general, one type of herpes (herpes simplex virus type 1, or HSV-1) causes sores on the mouth, while another type (type 2, or HSV-2) causes sores in the genital area. However, both types can cause sores in both places and can be spread from one region to the other via oral sex."Herpes sores typically affect the lips on the vermilion border [the line between your lips and your skin], but they can occur anywhere in the genital area," says Dr. Iglesia. "They’re ulcerative [they have well-defined borders] and painful." Although cold sores tend to appear on the outside of the mouth, Dr. Iglesia explains that they can also affect the tongue, inner lip, and eyes.If this is your first outbreak, you may have other symptoms , too, including a fever, muscle aches, and swollen lymph nodes. However, the virus doesn't go away when your symptoms subside; it stays dormant in your nerves, and sores may resurface. Those subsequent outbreaks can be triggered by factors such as stress and the hormonal fluctuations associated with your period. But they're not always easy to predict.Luckily, cold sores usually clear up on their own in a few weeks. But if you've noticed any sores that you think might be related to herpes — especially if this is the first time — definitely check in with your doctor to get an accurate diagnosis . There's no cure for herpes, but your doctor may prescribe medications that can shorten outbreaks and help prevent new ones, Dr. Iglesia says.However, most people with the virus have no symptoms whatsoever, which makes herpes much more common than most of us realize — and makes our regular gyno appointments all the more important.