What is wrong ?

Okay I'm 19 years old for starters. 
So my period is 20 days late which is very unusual for me I'm always right on time. I did a pregnancy test when I was just 10 days late and it was negative so I'm not pregnant what else could be wrong. I get really bad lower backaches on the left and I get cramps like period cramps but it's just not coming. Has anyone ever experienced this and if so what was wrong with you.