Concerned ... Got my vvf bfp on Saturday(12pdo) with a first response test.(my last one) went out and bought the .88…

Concerned ...

Got my vvf bfp on Saturday(12pdo) with a first response test.(my last one) went out and bought the .88 Walmart cheapies and clear blues to take over the next week to monitor my levels... Both yesterday morning and today I have been getting bfn... Af due date would be tomorrow. Has anyone experienced this? Could it just be that I'm not using first response?...I'm praying it's not another chemical. ?