What Causes Cramps In Abdomen And Legs?

I had my last period on March 9th (First day of my period) Now I am getting cramps in my stomach and legs for the past 3 to 4 days I did take a home pregnancy test twice but both were negative. I wanted to know if the tests taken was too early and should I wait till I miss my period which is die by first week of April. My cycle is of 28 days.