Most sensitive...

Here's the run down. I misscarried in December. 
I took the test posted here yesterday. It popped up after ten minutes... BUT, when I was pregnant it did the exact thing. 
I think I am super early on catching that I'm pregnant. 
No idea how many DPO I am, today was my due date for my period. Nothing so far. 😊
Now I'm wondering...
What is the most sensitive test you can take since I'm so early? This was Walmart here