Ovulation Tests

Hi, 

I bought the pink ovulation digital reader yesterday and on the first test I got a solid smiley face which then disappears even though I thought it was meant to stay for 48 hours? I’ve then tested a few more times and for a blank circle then another solid smiley, then more blank circles and then another solid smiley. Can anyone advise whether I’ve just got a dodgy test or what is going on? 

Thanks, 

Laura