Could I be pregnant

So my period is late by 3 days and I've taken 4 test and they've all been negative. Does anyone have any idea what could be going on. I'm on the pill and we use backup too (I'm a surrogate and can't be pregnant right now) and I don't know what to think. My period always comes on time; like clockwork. Cramping but no bleed. HELP!! 

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