very confused! this is a bfn?

so I took a hpt. Not at all expecting it to be BFP.. ( my temps are too low.) so I think I see a line and took it out of the case I really see a line, put it back in and the line seems to disappeard... This was 5 mins after I took the test. ( also this is a test that is exposed to air so removing it is ok!) Has this happened to anyone ??