what does it mean when the lines on a pregnancy test get lighter

Kr

My first positive tests!

Here are the ones I just took

What dose this mean my pee was light when I took the last two pregnancy test

 Another thing I did an ultrasound on Wednesday and I’m supposed to be eight weeks and I’m measuring less than five weeks they don’t know how many weeks I am because they said the fetus is so small they can’t measure it so I had to go back in two weeks this worries me