What does a dull ache under my armpit indicate?

There are several reasons why you might feel a dull ache under your armpit. One explanation is that a nerve in your upper chest or back may be irritated or squeezed. When nerves are affected, they can send dull, lingering pain into the armpit. Another possibility is muscle strain or overstretching of the tissues in that area, which might occur after exercise or a minor injury. In some cases, a condition known as axillary web syndrome can develop, with tight, rope-like bands under the skin causing discomfort even though you might not see a lump. Very rarely, a dull ache can be a signal of an internal problem, like the early spread of a tumor, though this is uncommon. It is good to remember that the presence of pain by itself is not always a bad sign, but if the pain continues for a long time, for over 2 weeks, gets worse, or comes with other changes like skin changes or swelling, you can check your symptoms to better understand what might be causing your discomfort and whether you should seek care from a healthcare provider.