Help?

Hi! I just turned 20 and usually my periods are pretty spot on with this app. So, my period is currently late by 3 days but I'm getting cramps and been moody and having cravings like I typically do when on my period. I also get that feeling like there's blood. But there's nothing there. I haven't had unprotected sex and tbh my bf doesn't ever finish in me just to be safe. So there's not really any chance that I could be pregnant. Does anyone know why I would be feeling like my period but no blood? I'm really starting to worry and I was hoping to find some help here.