Faint lines for a week- Does this make sense?

About 3 weeks ago I had unprotected sex (4 months postpartum). 1 week ago I started seeing faint lines on pregnancy test (VVF!). Today I expected to have a digital positive and know for sure, but instead I'm STILL just pulling super faint lines. This is my 4th pregnancy (1 miscarriage, 1 baby boy, 1 chemical), so I know what it feels like and I do feel pregnant. I guess my question is- is it normal to pull faint lines for that long without much progression of the darkness of the line? Isn't it odd that I had sex 3 weeks ago and it still isn't a BFP??