Why is BFP so faint?

Melissa • I`m a newlywed who just got married August 23, 2014. My hubby and I have officially been together for 4 years and are trying for our rainbow baby. We lost our first at 8 weeks in December. <3

I'm 3 days late and my tests look positive but the second line is so light. Today's looked similar to yesterday.

I want to get excited but I'm scared. I had a mmc in december.

What could cause this? Is it really a positive? I didn't want to make a dr appointment this early.