Disappearing positive pregnancy test!!!

[Roseybabe86]

Hey all I took a test this morning (my first morning wee) and it came up positive as soon as I put the cap back on the test, then after about 5mins the positive result was gone!! This has never happened to me b4 I'm a mother with 2 children so far and it has taken for me to be 3-4weeks pregnant in the past with both my babies b4 I got a positive result, has this ever happened to anyone??
I'm confused as not sure what to think, please get back to me thanks

(Oh it was a clear blue pregnacy test that I took)