My period is 29 days late. I have tender breasts and lower back pain. ALL PREGNANCY TESTS ARE NEGATIVE.?

Hormonal imbalance. You may wish to get a blood test for HCG level to be sure that you are not pregnant. Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. You may consider implanted contraceptive or an IUD.
Consult this site for more information on this topic.
http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.aspx
Practice safe sex, if you have sex.
Get HPV vaccine.