Am I pregnant?

Hey guys, I'm having trouble trying to find out whether or not I'm pregnant and I was hoping you guys would help me. I have taken four pregnancy tests since the thought occurred to me that I might be pregnant. However, all have come out negative. I didn't think I was pregnant until recently when my stomach started getting hard. The reason I didn't believe I was pregnant was because I have gotten my period these pasts months so the only symptoms that might suggest that I am pregnant are a hard stomach, constipation, and fatigue. Do you guys think there's a possibility I am pregnant? I appreciate everyone's opinions.