Left upper muscle discomfort is often due to strain or spasm, and you can usually get relief with rest, warm compress, and simple pain medicines like Paracetamol or anti-inflammatory drugs like Ibuprofen (if you have no kidney, stomach, or heart issues). A topical gel containing Diclofenac can also help when applied locally. Avoid heavy activity for a few days and do gentle stretching. However, if the pain is severe, persistent, or associated with chest tightness, breathlessness, or radiating pain, seek medical care to rule out other causes.