It could also be a matter of your urine being less concentrated than at first. If the temperature has changed, the amount of water you are drinking has changed, levels of physical activity... can all effect this.

I think the thing to remember is that with HPTs a line is a line, it's a question of whether it is there or not, nothing else. It's only with OPKs where it matters how dark the line is.

I'd certainly be getting a follow up blood test to check on things though. I hope it goes well for you.

BW