Who knows how BBT works!

I keep posting my chart everyday because it's really confusing to me. I get the general idea but my temps will go back and forth so I don't understand. I'm late for my period, I took a test the other day & it was negative. My uterus has been twitching, which it does here & there, but it's happening more, not really crampy so I'm lost. Can anyone help me figure out what's going on?