What Causes Nausea And Delayed Periods While UPT Is Negative?

I'm 23 years old,for the past week I've felt extremely tired, and I'm nauseous through most of the day, I'm three days late for my period (which never happens) I have cramping that last only a couple of minutes at most sometimes just seconds. I have taken three pregnancy test , all which came up negative. I don't know what else could be causing all of this.