My period is due tomorrow..

I have no cramps what so ever like I usually get the day before my period is due! I'm feeling ill at the moment but have a cold so I'm not holding out much hope that I'm pregnant and I'm probably run down. I did a test this morning as according to glow in 12dpo it was negative.. I just did a pregnancy week calculator on first response website by the date of my last period and cycle length it says id be 3 weeks and 6 days pregnant say I was is this the reason I'm getting a negative test? (It's blue dye Tesco brand) is 3 weeks too early to pick up a test? I know I'm clutching straws but I'm feeling desperate!