My opinion (just mine), is dont lose hope! As far as I know, the majority of women that have perfectly normal pregnancies and healthy babies don't take tests a week after getting their original BFP, I'm sure that a few do but not the majority. So therefore, as far as we know, perhaps it's perfectly normal for test lines to get lighter?! I know I probably sound like a blabbering idiot but the fact is none of us know for sure!
Go to your docs and either express your honest concerns or if you feel silly doing that say you have been suffering pain (I know it's terrible to lie but you're obviously going out of your mind!) I've got everything crossed for you, just don't lose hope, remember that everyone is different and there's no conclusive evidence on this xx