Is this a chemical?

Hi everyone! Im 42 and we have been trying for our 2nd for 4 years now. Ive just had an unsuccessful ivf cycle and then 2 months later, I tested on my period due date and got a clear positive in the afternoon. By Monday the line was darker but still not as dark as control line. All was well. Im now 5 days late and my test has suddenly faded over 2 days to barely visible. Im on progesterone but have no current spotting or cramping. Should i assume this is a chemical or can a line magically get darker again? (here's hoping!) The line is definitely stronger in real life but still very faint.