Shortened bleeding as you describe is fairly typical at your age.
It is plausible to have regular non-
niddah spotting before the flow begins and not to become niddah from it. For more information about this, please see our site's articles "
Toilet Paper" and "
Ketamim."
Please note that we advise women to
abstain from relations for 24 hours when they have non-
niddah spotting, as a precaution against having a flow begin during relations. This is not a strict halachic requirement, however.
The five day minimum begins when a woman becomes
niddah, or acts as though she is in
niddah (including abstention from relations and all the
harchakot) out of doubt. (For more information, please see our article "
Beginning the Five Day Minimum".) A woman who has short flows and is interested in beginning the five days early may sometimes choose to perform a
bedikah in order to begin–and therefore end–niddah status earlier. Even that might not work when the color is a
non-niddah shade of brown.
In short, being "lenient in two directions" doesn't work in cases like yours. The five-day minimum isn't shortened; it is just begun either earlier or later, with very limited control over the timing.
At this point in your cycle, your flexibility is limited. Perhaps you could revisit the possibility of immersing on Friday night. Otherwise, if your husband agrees, you can
delay to Saturday night.
Please write back with any further questions.
