Hi,
Although there is a very slim change that you could have been pregnant and had a spontaneous abortion, the chances of this are exceedingly small. More likely, you are probably entering the first stages of menopause. The hormonal fluctuations caused by pre-monopause cause changes in your menstruation, as well as the glands in your breasts which can cause tenderness. This is much more likely than pregnancy, considering the vasectomy.
I recommend you get bloodwork by your doctor, including an FSH and LH, as well as B-HCG. These will show if you are entering menopause, as well as rule out any "products of conception" that theoretically could have stayed in your uterus - although unlikely.
You will feel better getting a definative answer, so get this blood work. Also don't be surprised if future periods are also inconsistent, along with some spotting. This is to be expected if you are peri-menopausal.
Good luck and please click ACCEPT if this information was helpful. Thanks.
Doctor
Board Certified Internal Medicine, Former Chief Resident, Yale New Haven Health