Test Not Getting Darker? Walmart Brand..

Kayla • I`m 22 years old, married, have an 19 month old son, and due March 14th with baby #2.
So I'm paranoid about having a chemical like I did before my son, so I've been taking a test everyday.. Except today. I only have one left and I'm saving it for tomorrow. Anyway, it's always positive but the line isn't getting darker.. Should I be worried?