Reasons my period could be late?

Obviously besides pregnancy.

As of today my period is six days late, I was scared I was pregnant, I've been rather crampy, my nipples are sensitive and sore if you touch them, been tired, and even a bit nauseous to the extent of (gross) throwing up in my mouth a little throughout the day. Well, I've take four pregnancy tests (two digital and two blue line) and nothing!! So okay, what are some other reasons it could be late? Could it just be too early to tell? Like there isn't even a faint line I don't think. I just don't wanna be like hey boyfriend my periods late and I'm worried I might be pregnant and stress him for no reason? Or get his hopes up.