When did you ovulate? You need to be guideed by when you ovulated as opposed to when your AF is due.....there's utterly no point testing until 8dpo at the very earliest, more likely 9 or 10. There simply isn't enough HCG in the body at this point to register on any tests - no matter how sensitive.
If you did indeed get pregnant, the fertilised egg needs to implant, and only then will your HCG levels start rising,, so if you ovulate early....at CD8 or 9, then yes, you theoretically could start testing now. But if you ovulate in a normal window of say CD13 to 17, there's pretty little chance that even if you were pregnant that you would get any line at all.....you've probably not even gone through implantation at this point.
It can't hurt your chances to test early, but it can seriously hurt your purse, and believe me, it can be soul destroying to see the negative result!
I'd try and hang on....I know its not easy, but unless you know you ovulated at lest 8 or 9 days ago, it would be impossible for you to get a positive result, even if you were pregnant.
Good luck x