ok so my period is almost 2 weeks late.... can someone tell me the other reasons for a missed period? and before you ask... no I'm not stressed nor do I have any medical problems... are there any other reasons? I took a pregnancy test 5 days ago( it came out negative )so I've canceled out pregnancy, but are there any other reasons?

if it helps to know, I've been quite moody and agitated. also I have period like cramps throughout the day( no blood obviously lol)... plus my back hurts.