What Causes A Missed Period With A Negative HPT?

I had a tubal ligation 2 1/2 years ago during a csection birth of my daughte at the age of 22r, for the first time ever I have missed a period(other than being pregnant) I have a sudden outbreak of acne, abdominal pain and cramps the day I was suppose to start I had severe back pain that disappeared the next day, its now 2 weeks late and still no AF I woke in he middle of the night last night with severe nausea..Took a HPT at 1 week late and it was neg...Could I be pregnant? Or what could be wrong