I am due my period on Sunday and tested twice today using Tes*o brand pregnancy tests.
When I replaced the cap and layed the test flat, a pink/purple positive line came up immediately, as the urine was passing through the window. Obviously I was ecstatic for a few seconds!! The control line then appeared but then the positive line in the test window disappeared completely 
I kept the test for ages and no positive line was visible, not even a shadow - no matter what way I studied and tilted it 
I re-tested with the other test in the pack next time I went to the loo and the exact same thing happened again. The line disappeared in seconds and is (almost 2 hours later) still a BFN.
Very confused. I am somewhat experienced at POAS and never came across this before.
Has this happened to anyone else? I am 99.9% sure it's obviously a negative but just clinging to the fact that there was a pink line there...
Thank you all in advance, I am going crazy!