A positive test but then a negative ? Confused

Darcey

So yesterday I took a test it came up with 2 pink lines, one was dark and the other a faint line, it says on the packet this is still a positive. So that night I then tested with a clear blue but it came out negative ... so confused. Then this morning I tried the clear blue digital and again in the results bar it says ,' not pregnant '. Has anyone experienced this before ? And been pregnant ?