Congratulations on your mama instinct, with two kids already, that feeling is usually telling you something! A faint shadow within the reading window (usually 3-5 minutes) does lean more towards a positive, especially since your hCG would still be very low this early, so the line naturally appears very light at first. Do take note though, if the line only appeared after you left the test sitting for 15-20 minutes, it may just be an evaporation line, so always read within the time stated on the packaging. My friend tried testing a few days before her period was due and the line was barely there, but by the next morning when she retested it was noticeably darker, so your plan to retest on the day of your period is actually a very sensible approach. Wishing you clarity soon, and do rest well in the meantime, your body is clearly trying to tell you something.