4 tests, 2 positive and 2 negative

So I took a test and it came back with a very faint line, so we took another one to confirm it and that also came back with a very faint positive line, ok so now I'm getting excited I've waited years for another child, the next day I go and get another test in hopes that the line would be a little darker, and it came back negative! I couldn't believe it! So I took another 1 a few hours later and again negative, I've still not got my period and I'm 3 days late for it... Amy ideas on what could be going on? TIA xx