HELP! Second test line lighter????

Ok so yesterday afternoon I tested and got a very clear positive with a FRER test within 30 seconds. I just tested again with FMU expecting the positive line to be darker and it's actually fainter. I already have a doctor's appointment scheduled for Tuesday but has this happened to anyone else?? Is this a bad sign?? AF is a day late and I am 15 DPO. Shouldn't the tests be getting a stronger positive day by day? I'm worried now. :/