different days for af

Hi ladies I need help. I use Ovia for more then 3 years it shows my period coming in 4 days. Glow shows my period coming in 7 days. I took a waltmart test and it was negative. I'm having horrible nauseous especially in the mornings, and now cramps. Can be a sign of pregnancy? I can't remember how was with my kids, what I remember is that I took a first response 6 days before my period and they both were positives. That was long ago. Now we are trying for our rainbow baby after 2 years. My nauseous make me think I'm pregnant but I'm really scare. Thanks