The mishna lists specific types of vestos ha'guf that if they occur repeatedly (tosafos) prior to a woman getting her period, even just for one cycle, she establishes a veset (according to Rebbi that normally a veset takes two times, here it only takes one). The Mishna explains the definition of vestos ha'guf to be thing such as sneezing and yawning repeatedly prior to the start of the period. The gemara then identifies another situation where a woman eats garlic or onions and then gets a period. If this were to happen 3 times she would establish a veset. Tosafos question why the gemara considers it possible to establish a veset for eating garlic, yet the gemara says 11a that a woman cannot establish a veset for jumping at the time of the bleeding? Tosafos explains that jumping is an "o'neis", meaning a very mechanical method of causing the bleeding and therefore doesn't establish a veset since it is not so predictable. Whereas eating sharp foods is a more chemical cause of the bleeding which is more predictably associated with the bleeding. The Rashba in the first perek distinguishes between jumping and the standard vestos ha'guf of the mishna. A veset is an indication that the bleeding is about to begin, meaning that the symptom is caused by the start of the menstruation and therefore an indication that the bleeding is about to start. However, jumping is not a sign or indication of the bleeding about to start, it is actually the cause. A veset is only when the bleeding causes the symptom or action, not when the action causes the bleeding. This approach doesn't answer Tosafos question where the sharp foods would seem to be the cause of the bleeding.
In the next Mishna we pasken like Rav Yehuda who says that a woman must be concerned for her period the entire o'nah (12 hour time slot) that she is expecting it, either by day or by night. The Rashash asks that since she has to expect her period our before it usually occurs, from the start of the o'nah that it usually occurs in, why do we say that a woman who has a veset is דיה שעתה and is not tamei retroactively? Shouldn't we be concerned that the blood was here a few hours prior to the finding of it from the start of the o'nah? The Rashash answers that the concept of forbidding a woman to her husband during the o'nah is out of fear that the chimud for being with her husband would contribute to the blood coming earlier. Therefore, when she gets her period she doesn't have to be concerned that the blood came earlier, but if she were permitted to her husband at the beginning of the o'nah we would be concerned that the blood will come earlier. The chimud can actually be a cause of the dam, not just a halachic prohibition created by the dam.