Hello and Welcome to MedHelp's Pain Management Forum. I am glad that you found us and took the time to post... but very sad to hear that you cannot find a diagnosis. We know about Chronic Pain, even undiagnosed Chronic Pain and the havoc it often reaps upon our days and the sleep it robs from us at night. My heart goes out to you!
Have you consulted a gynecologist? I don't know your general age or generation. When I was in my late twenties I experienced very similar pain to what you are describing, also on the left side. I was finally diagnosed with what they called Fibrocystic Breast Disease. They have taken the "Disease" portion off the diagnosis now as it is considered a condition and not a disease. The pain was so extreme that at times I could not lay on the effected side. The pain would wake me from my sleep if I rolled over on my left side. The pain in the axilla (under arm) area was just as extreme.
I am not saying that is what you have.... I am just wondering if it has ever been considered?
Axilla pain can also be related to a muscle strain. You don't say how long this has been going on. Axilla and breast pain can also be related to nerve entrapment. In those instances it's known as referred pain, meaning originating in one location but (felt) referred to another area. Have you had an MRI of your Thoracic Spine? This would provide a detailed picture and reveal any possible structural issues such as DDD (Degenerative Disc Disease) or a bulging disc.
Is it fair to say that your PCP has not done an extensive investigated into your pain? Or have you had multiple tests and exams? Have you seen medical specialist? If so, which specialties? Additional information may be helpful with our suggestions to you? Do you have a palpable lumps?
Rather you add more info or not, remember you are your own best health care advocate. Be assertive and insist on answers to your pain. No one cares more about your body then you do.... and it's sending you signals that something is not right. Don't ignore it or allow your PCP to dismiss your symptoms.
I hope you will keep in touch. I'll look forward to your updates with interest.
Best of Luck and Take Care,