Could I be pregnant? Boyfriend had a vasectomy 3-4 years ago.?

Is it possible that I could be pregnant? I am having an unusual tenderness in my left only (upper-above the nipple.) It has been sore to the touch for 4 days now. I have also been having alot of headaches and los of appetite (could be contributed to stress.) If you consider a 28 day cycle then I had twice around my date of ovulation and I should start my period on Friday. The kicker here is that my boyfriend had a vasectomy between 3 and 4 years ago (when he was married.) I would really love a child - he doesn't want one. Could I be pregnant? If not then what is the pain about (never experienced it before.) Is it too early to test?