Feel under the armpit, too. Change hands and examine the other breast. Also, repeat these procedures while standing up. If you find any lump that's unusual, see your doctor within a few days. Most breast lumps are not cancer, but this can't be ruled out without medical testing.
Some women are reluctant or fearful to examine their own breasts.
If you can't seem to perform self-exams, consider visiting a clinic for an exam on a regular basis. Early detection is vital, because the earlier breast cancer is found, the higher the cure rate tends to be.