"Lemai? Ilema Leyamim - Ha kol Yoma DeLo Kafits Lo Chazay."
My question is: what makes the Gemara understand that if she didn't "see" on (say) Monday, its because she didn't jump on Monday, maybe it depends only on days and the reason why she didn't "see" on Monday is because the day was changed (-- as before she "saw" only on Sundays)?
You appear to be reading the words, "Ha kol Yoma DeLo Kafits Lo Chaza'i" to mean, "But on Monday she didn't see a show of blood." That is to say, if a woman jumped and saw a show of blood 3 times on Sundays, it cannot create a Veses for bleeding on Sunday without jumping since she did not bleed when she didn't jump on Monday . If this interpretation is correct, your question is certainly a solid one.
However, I understood those words to mean, "But on the Sundays on which she didn't jump, she didn't see a show of blood," which clearly proves that the day alone, without the jump, does not cause bleeding.
You probably didn't read the words this way because the Gemara never mentioned any Sundays on which the woman did not jump (it only mentions three Sundays on which she did jump). Nevertheless, I maintain that the above reading is correct and it is referring to the Sundays before the one on which the woman started to jump.
Thanks for sharing your questions with us! Be well,
The "Reshash" says like you that it refers to the sunday before she started jumping, "ayin shom".
Thank you, Boruch!