What Causes Delayed Periods Along With Cramps?

I am 20, sexually active femaile. I was supose to get my period either the 15th or 16 and now its the 18th and no period. I have been having cramps without blood untill today when i went to wipe i had about half a T spoon of blood on the toilet paper.. been feeling very weak and tired. and i haven't been hungry which is very odd for me.