Missed period while on the pill, but negative pregnancy test?

I have been on the pill for 10 years- minus the 2 times I was pregnant with my daughters, 5 months while I breastfed, and about a year when I was trying to conceive. I have always had a very, VERY regular period. However, I am currently 6 days late. I have taken at least 5 home pregnancy tests across different days and they have all been negative. I was worried a couple of weeks even before my period. I had one day of spotting. At the beginning of the month I missed 4-5 pills in a row (very rare for me to do.) Because of my past pregnancies and my trying to conceive, I am very aware of my ovulation cycle. After drawing it out, I did have sex around the time I ovulated and the spotting was about the time implantation occurred. Now my period is late by almost a week. Is this one huge set of coincidences? I have never missed a period except for when I was pregnant. Oh, within the past 5 months I had seen an endocrinologist. I was negative for polycystic ovary syndrome and my thyroid levels were normal. I have not had any dietary changes, no extra stress, and have not been on antibiotics. What could be going on? Despite the tests, could pregnancy still be possible?