What are the possible reasons for sharp pain under my left armpit?

There are several reasons why you might feel sharp pain under your left armpit. Sometimes, the pain comes from an irritation or pinched nerve running through the area. This can happen if you strain your shoulder or arm muscles or if there is inflammation around the nerve. In other cases, your lymph nodes could be reacting to an infection or inflammation, which might cause pain in that area. Although it is uncommon, sharp armpit pain can also be an atypical sign of a more serious issue like cancer. For example, there have been cases where breast cancer or even gastric cancer that has spread (metastasized) to the armpit area has caused pain without an obvious lump. These are rare occurrences but are important to consider, especially if the pain does not go away or is accompanied by other symptoms such as swelling, changes in the skin, or unexplained weight loss. If you experience persistent or worsening pain, you can use Ubie's free AI symptom checker to help identify possible causes, and it is best to consult a healthcare provider for an evaluation to determine the exact cause and appropriate treatment.