Dear sjojo: It is not uncommon for an assymetric density to require an additional film to further clarify. Often, this turns out to be tissue folded over on itself and will smooth out with compression. If the density remains, an ultrasound may be done and further evaluation determined from there. Most breast cancer is not painful. In terms of the discomfort, this would need to be evaluated by your doctor. We cannot speculate on what may be contributing to this problem. Certainly, stress can contribute to discomfort but it would be important to rule out any causes.
I haven't known stress to discriminate like that although I guess if you are anxious about symptoms on one side of your body then your mind is more likely to manifest them.
Breast issues aside, is it possible you have a pinched nerve or something like that that could be affecting other body parts?
J