pregnancy tests

I've taken 3 tests now and they are all faint but definitely a line there! Is it just because I'm too early or should it be getting darker by now? I'm going to get first response now and will test with that tomorrow. I've used 3 different tests aswell so don't think it would be faulty on all 3? If I am pregnant according to nurture I'll be 4 weeks today. My period was due yesterday/today? Would any of that make a difference? I'm just worried as we want this so much and I'm worried that a faint line isn't what I think it is maybe? Even though I took a test last week and it was clear negative? Any help appreciated