I have pain under my arm when I move it/ pick up something heavy. It’s been there for a few weeks. It feels swollen at times - like I’m holding a tennis ball under my arm. It doesn’t feel swollen at all to touch/ there is no visible swelling and no lumps. What could this be? All I can think is breast cancer. I have quite a sedentary life style due to other health issues so I would have thought unlikely I’d have strained a muscle or whatever