Thank you for your question. Let me first reassure you that the vast majority of swollen lymph nodes soon return to normal size on their own without the need for an antibiotic and do not indicate a serious or dangerous medical condition. Having said that, let's talk about the reasons lymph nodes get enlarged and when you may need to be concerned.
There are over 500 lymph nodes (also called lymph glands) in the body, which are connected in a network called the lymphatic system. Even though they are about the size of a pea, lymph nodes are usually not noticeable unless they are close to the skin's surface (such as in the neck, armpit or groin) or are much larger than usual.
Lymph nodes contain white blood cells and filter the germs in the body. Some nodes can become enlarged when they work overtime during common infections such as the common cold, strep throat, infected skin wounds or canker sores caused by a virus, as you may have had. An enlarged lymph node in the area of an infection (such as a swollen neck node during an ear infection) may double in size, going from about a quarter or half of an inch to nearly an inch wide. The node will usually shrink back to normal within a few weeks after the infection has passed.
A lymph node itself can get infected when overwhelmed by the infection it is trying to fight. The node may become swollen and tender to the touch, and the overlying skin may be red and warm. Infected nodes (called "lymphadenitis") are fairly common and may be treated with an antibiotic, warm compresses and over-the-counter pain relievers such as acetaminophen or ibuprofen. If untreated, infected lymph nodes may progress to an abscess (pus collection) or spread bacteria to the overlying skin or into the bloodstream. This may be what your doctor was referring to.
Less common causes of enlarged lymph nodes include infections such as cat scratch disease, tuberculosis and sexually transmitted diseases. Certain conditions such as rheumatoid arthritis and lupus are also associated with enlarged nodes. Finally, cancer and HIV infection are rare causes. In one report, the chance of a patient having a diagnosis of cancer after showing up to their primary care doctor with an enlarged lymph node was around 1 percent.
For enlarged lymph nodes that persist (longer than about two weeks), get bigger or happen along with other symptoms such as fever, unexplained weight loss and night sweats, your doctor may order blood tests, X-rays or other studies to try to determine the cause. A biopsy of the node may be needed if a cause is not found.
Good luck, and please check in with your doctor if you have any questions!