AF 13 days late but negative tests.

Hi all, just looking for some advice if possible.
AF is now 13 days late, I'm usually very regular, have been ttc for 2 years with no luck, my right ovary is polycystic. Taken 2 tests so far; both negative but feeling nauseous and peeing more frequently for the last week and today have actually thrown up 3 times. I'm very bloated and getting none of the PMS symptoms I usually get before AF makes an appearance.
I'm convinced that I'm pregnant, could I be getting false negs because of low hcg levels (my mum tested negative 6 times and AF was five weeks overdue before a pos result) or could I be experiencing these symptoms psychologically because I've convinced myself that I am? 
Does anyone have an experience with false negatives when they were sure they were expecting?
Any help much appreciated, driving myself insane!😫😫

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