First positive and then negative..

Hi everyone!
I don't understand anything anymore.. On April 8th I had a positive ovulation test. So I should have gotten my period last Wednesday, but up until now nothing has shown up. I did a test on Wednesday too and at first I saw nothing, just me and a friend (also from this forum :-D) and my man. We did see a faint line though. So I did another test on Friday afternoon and that one was positive (not very dark yet but definitely clear). So I was happy! But then today I did a digital test and that one was negative. After that I immediately did another test (the same one as the one that was positive on Friday) and that one was also negative. I don't understand anything anymore. Still no signs of getting my period. I'm feeling a bit down because of this. And I don't know what to do anymore.. Since this afternoon I've had painful twinges in the lower part of my back, sometimes I can't even walk. Does this mean I'm going to get my period after all?

Love, Manon