Positive then negative test one hour later

Using FMU my test was positive with a VERY faint line. Of course this was my first cycle using femara so I was over the moon. I took another test about 45 minutes later and it was negative. Not due for AF for a couple of days. Could the second test have turned out negative because the HCG levels weren’t high enough. I know most people say to use FMU because you have more concentrated urine. Do u think its possible I tested way too soon right after i took the first or could i have gotten a false positive? Any similar experiences?