Positive test and then negative test

So been trying to conceive for about 8 months. I have irregular periods so knowing when I'm ovulating is tricky. This morning, I took an asda's own brand with first morning wee and got a pregnant sign. Over the moon!! Then I bought a clear blue test which can tell you how many weeks and tested about 2:30pm and it said not pregnant... I'm really confused. Has this happened to anyone? And did you find out you were pregnant? I am going to do another clear blue tomorrow morning. Fingers crossed.