Line appears then disappears

Nattie • N V Cull
hi guys, I get my own £3.50 tesco brand pregnancy tests, now I'm a little worried. 
I did 6 before and they came up with very thin blue lines, 
Well I went to clinic Wednesday and she said she could see a pink line forming. And it was so so faint. 
Well I'm 6 days late, and I just did a test off tesco brand and it had a thin blue line which I read in 3 minutes and then disappeared can only see it if I shine light on it, and the same happened last night. What does this mean?