18dpo 3 days late!

Im 18dpo i used opks to track the big o. I was due for my period 3 days ago.  Still no AF and a BFN on 16 dpo. Fertility friend says for me to test tom. But I would think being on 18dpo it would show up I normally have 34ish day cycle today is cycle day 41. Any advice? I just wanna start or be preg I just started feel sick at my stomach today but I haven't ate much today either but that not unusual for me. We are really just at work in the mornings.