Amenorrhea or pregnancy?

[ellisbell]

Hi,
I've had unprotected sex frequently enough during the past month, but he always pulls out in plenty of time. I'm three days late. During this month, I've very abruptly increased the amount that I exercise. I've been running a mile every day after not having run at all for a year and I've been lifting much heavier weights 4x a week. I took a test last night (end of day 2 of being late) and it was negative, but it was kroger brand. I'm not really feeling any kind of symptoms, and I didn't notice any implantation bleeding. I'm pretty sure my period is late/absent because of the exercise, but I want to be absolutely positive about it, and I'm not sure if that relatively small amount of exercise (a year ago, I was training much harder) could cause amenorrhea. Have I tested early enough that there is a significant chance of a false negative? If so, would a blood test be accurate at this point?