Could I still be pregnant

Hi all I need some advice I am due af on 28 August I no I ovulated according to opk and cm 
I had cramping all over last weekend n still have the odd twinge now and again
I feeling tired and got headachy sicky  feeling 
Loads of cm since ovulation 
I never get like this before af 
Took 4 tests all been negative 
Help !!!

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