It is normal for menstrual flow to contain the stringy black things you have described. They are probably small blood clots.
Well there are lots of types of fins for betta fish. my betta fish has a long stringy tail and fins, but my female has short average fins. most have plain fins. mine is the only one i have seen in my area with a stringy tail. most fish have stringy tails if they are sick but not mine he was bred to be like that. depending on the fish and the area well there are lots of things that are considered normal for bettas.
Fiber or maybe a worm
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they are adapted to their job because they have stringy things
the long stringy things at the bottom of the jelly fish are the tentacles
Biceps femoris
kat kins
to me that sounds like worms. not to scare you or anything but i would go to the doctor right away even thought it may be embarressing. and also . if your faeces are bloody go or call doctor immediately .
could be worms, or perhaps a liver problem.
Although strange things can happen, it would be extremely unlikely given the combination of a very long cycle length and the apparently normal period.
Many things can change the flow of your period, including stress, illness, and weight gain or loss. If it worries you see a doctor.
What may be normal for one woman, may be different for another. Many things affect how you flow; whether it's stress, pregnancy, etc. Keep track if you have your period last for one day again.