Am I late or Pregnant??

Chair
So usually evrey month I am right on track to start my period the day I am supposed to. My period always comes at the end of the month sound the 25th-30th. 
This month I was due to start on Monday the 24th and didn't. It worried me a little because I know my body and it always starts in time. I talked to my older sister and she told me to wait a while and take a pregnancy test. I look one Tuesday and it was negative and Wednesday morning too and it was also negative. It's Friday the 28th and still haven't started.
Yesterday I felt nauseous and like everything I ate I was going to puke and  just exhausted! 
I'm going to take another test on Saturday morning (tomorrow) if I haven't started. 
I want you guys opinion on what you think is going on, am I pregnant? Or am I just late?