I am having pain over my left breast going into my arm pit. It feels like a deep sore.

It would be difficult to further assess this pain without knowing your age and other symptoms, but it sounds like you should be evaluated by a physician. Muscle strains, breast infections, and even heart problems can cause the symptoms you’ve described, and a conversation and physical exam by a physician would help narrow down the list of potential causes of your pain. Fortunately, things that cause breast pain are often benign problems, while detecting one or multiple painless lumps would be more concerning. It’s a good idea to perform a “self-breast exam” (SBE) prior to an office visit so you are prepared with any questions that arise. Please refer to the following website for more information on performing an SBE.