Can you test sooner than it says on the box?

I should be a pro at this since we are trying for #4. But in sicker than a dog and showing all the signs. I had a tubal ligation almost 2 1/2 years ago but I had a 16 week miscarriage 2 months ago (didn't even know I was pregnant and hadn't had a period in 8 months) I've had two regular periods right on time back to back and 18 days til my next one. I'm still in my ovulation week but I need to know if I am sick sick or pregnant