What Causes Delayed Menstruation, Sore Breasts, Nausea And Lower Back Pain?

I am over a week late for my period. I have been on the pill but I don't take it regularly and have skipped periods. I have had all negative home tests and one negative blood test 4 days ago. I have sore breasts, nausea, enhanced smells, lower back pain (which is normal for me), I'm tired and all I want to eat is oysters. Normally after I stop taking the pill my period is there 3 days later. It's now over a week and no sign of my period. My question is could I be pregnant even though early tests are saying no. What else could be wrong?