How long after missing a period will a preg. test show positive?

I am not on day 30 and have not started, last month my cycles were 22 days apart. I have taken 3 test, 2 cheap ones and 1 First response. they are all negative, could I still be pregnant? Last week I was cramping pretty bad, which I had with my first preg, and this week Mon, tues I had cramps. When should I retest and how does the blood test work, would I have to have my blood drawn 2x over 48 hours?