19 DPO 1 faint positive and then a BFN

Tomorrow makes 3 weeks that I will be late... I had a faint positive this afternoon then this evening BFN. 😞 I scheduled a dr apt. I've thrown up the past two mornings.. I scheduled a dr apt to get a blood test.. But I'm wondering why I would get a negative.. Or if it was even a faint positive to begin with.