Was the test fully negative, or was there a very, very faint positive?
I had a urine test at a doctor's office when I was 4 1/2 weeks pregnant (a few days late for my period) and it came out negative. The blood test, however, was a loud and proud positive.
Like you, I ALWAYS have sore breasts when my period's coming, and like you, it didn't happen when I had conceived (though whoa nelly, it sure caught up with me at 6 weeks!)
Get thee to a blood test. And if it's positive, congratulations!