Somewhat new to this

Let me begin with saying that I have 2 kids, ages 7 & 5....but with both I was already 6wks pregnant & 3months along with I found out. Niether of them, I didn't have morning sickness.
My situation now is that in Dec I was 10days late & before my period finally started all pregnancy test were negative. When it did start, it only lasted a day in a half, compared to the usual 4 days.
My fiance' & I are now trying to conceive. The last 2 mornings I have had abdominal cramps & top of my stomach has felt tight...within an hour cramps have continued to my lower back then up between my shoulder blades. More home pregnanct test read negative. What's going on?