What Causes A Delayed Period With Negative Pregnancy Tests?

Hi. I'm 36 yrs of age. I have 2 children. Husband had vasectomy 02/11. My last cycle started on 09/21/11. I have pain in lower part of abdomen on right side. Pain comes and goes. More throbbing than sharp. I've never been late before other than when I was pregnant with other 2 children. I started my cycle at the age of 9. Takne 5 pregnant tests. They were all negative. Husband just went back to make sure procedure was successful. His test result was negative. I'm trying to wait until I'm al least 6 weeks past my last cycle before going to the doctor.