Anyone know what this means?

So my period is 4 days late. Typically the week before my period starts(last week) I start getting a little crampy and PMS, which I did. But my period never came. I'm still cramping like I do when I'm getting my period but my period still hasn't come. I haven't had any spotting or anything. Does anyone know why this is? I feel a little nauseous and tired as well. I took two pregnancy tests and they were both negative. I may take another in a few more days. Thanks for all answers in advance