Pain on the side of the breast

I think the advise re lymphedema or seroma are spot on. Also the tension you feel,when you stretch your arm could be something called cording. I am not sure what causes it, but if you feel around in your stretched armpit you may feel something that feels like a piece of stretched rope. My occupational therapist that I was seeing for my lymphedema taught me how to rub it between my fingers. If you stretch it hard enough you may even feel it go pop. I did and it freaked me out, but it is actually a good thing as you need to massage it to break the cording up. The simple exercise of walking your fingers up the wall as far as you can go, and going a little further each day really helps. If you are not sure, make an appointment with a oncology trained Physio/ occupational therapist. Good luck. Paula :)