Period is late, help!!

My period is late by three days, this rarely happens but this isn't my concern. Usually, when I PMS it's a good week or half week before my period and the only symptom I get are body cramps(mostly in my legs). Howevermy period was suppose to start about 3 days ago and since then I've felt like I'm on my period. In class(I'm 16) I feel cramps and like I'm bleeding out but when I go to the bathroom I'm high and dry. I've been having terrible headaches, fatigue and body pains just like I'm on my period but no blood. It's scaring me a bit. Also, I don't have any relationships nor am I sexual active but I do masterbate. 
Has this ever happened to anyone else? (Sorry if this sounds paranoid)

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