A large lump (more than 2 cm) appeared suddenly on my right breast. It was accompanied by occassional discomfort - as though a thumb was being pressed onto my breast. I immediately went to my doctor, who referred me to a specialist breast clinic. She did not think that it would be anything serious as I am 39 and I have none of the usual indications of breast cancer being a risk. I had mammograms, ultrasounds and biopsies, all within 2 weeks of finding the original lump. I have been diagnosed with DCIS, a form of early breast cancer. As it has been found early, I am in little danger from this and can have treatment which will "cure" me. What I am saying to you is that although the chances are that it is nothing (9 out of 10 women who go to breast clinic do not have cancer) getting it checked out sooner rather than later will asuage your concern. If it is something early treatment is good. If it is nothing (which it probably is) you can stop worrying about it and get on with your life.
Dear worried lady: In general, breast cancer is not painful. Of course, nothing is impossible. Some of what you describe sounds muscular. Regardless, if you have a large breast lump, it should be evaluated by your doctor or breast specialist.