Pregnancy tests

I am 4 weeks and 2 days pregnant and 6 days ago I took a first response early result and that’s what confirmed my pregnancy. I then took another one 2 days later and it got stronger.

I have now current time taken a first response rapid results test and it looks slightly lighter than the other ones I took. Is this normal? Is it to do with the sensitivity of the tests? I don’t know how different the sensitivity is 😑. After two chemicals I’d hate to think it’s happening again. I’m just very paranoid and don’t want anything to go wrong 😟 I would of at least thought it would get stronger.