What Causes Stomach Cramps Before A Period?

I stopped using the evro patch 23 days ago. Me and my partner are trying for a baby. My period should start tomorrow but I have been getting cramps on my right side of my stomach for about 3 days. Last night I couldn't sleep as I felt like I was going to be sick then the cramps was so bad I was in alot of pain. I did a pregnancy test yesterday that said it could detect pregnancy from 8 days after conception. Could I be pregnant or is it just bad pms. I used to experience bad pms before the patch but not cramps. Just back and legs hurting.