With the advent of ergonomic improvements in mammography units and improved imaging techniques, this factor is far less of a problem now than before.
Everyone experiences mammograms differently. Some women may feel pain during the procedure, and others may not feel anything at all.
Most women feel some discomfort during the actual X-ray process. The pressure against your breasts from the testing equipment can cause pain or discomfort but it will never be beyond what you are capable of dealing with.