Lines not getting darker

I got a bfp (on 2 different types of tests) at 12dpo. It wasn't dark but definitely there. Since then I've been taking multiple tests every day because I'm so scared it's a chemical. I am 15 dpo and they aren't getting lighter but they aren't any darker either. I realize it's still very early...according to this app I'm not even 4 weeks yet but should they be getting darker? Today I got a positive on a digital and I haven't had any spotting at all but I guess I'm just a worrier. When did your tests start to get darker?