One of the lesser well-known laws of Niddah, requires that husband and wife separate from sex on the day (or night) that the wife is expecting to get her period. This day is called the veset, and this requirement is learned from the verse "And you shall separate the Children of Israel from their impurity" (Lev. 15:31), which teaches that we must take safeguards to not unwittingly violate the prohibition of niddah. Now, this time, the veset, is determined in a formalistic way in the gemara - based on a three-time repeating pattern of a the length of the menstrual cycle (e.g., three months in a row a woman has a 28-day cycle) or the day of the Hebrew month (e.g., three months in a row she starts her period on the fifth day of the Hebrew month).
The problem is that the focus on formalistic criteria, as is particularly evident in the case of the Hebrew month, is not based on any real-world causation. [It should be noted that Ra'ah, 13th century Spain, stated that such a position was untenable, and thus reread the gemarot to be referring not to the day of the Hebrew month, but to the day from the actual new moon, as women's cycles are sometimes influenced by the lunar cycle. His position was not accepted.] In addition, a veset based on the length of the cycle can only be established when it starts either always in the daytime or always at nighttime. Thus, a woman with a regular 28 day 4 hour cycle would not have a veset according to these rules. What results is a system with a lot of formal rules, which are often difficult to remember and to keep, and which are often not good predictors of when a woman will actually get her period. To some degree we have lost the forest for the trees, and in focusing on all the specifics of vesatot have lost sight of the need to exhibit caution when the woman is actually expecting her period.
Consider the case when the veset passes and the woman does not get her period. Let us say that she normally gets her period every 30 days, and now it is day 31 and she has not had her period yet. According to halakha, there is no longer any obligation for her to check, and most poskim will be formalists here and say that there are now no further restrictions or requirements. But from a real-world-based approach we know that unless the woman is pregnant it becomes more likely with every passing hour that she will have her period. As a result, there are a few poskim who would advise checking before sex, or - as some women can tell when they are about to get their period - trying to be conscious of whether they feel that they are about to have their period.
While in a case of a normal period most poskim would follow the formalist approach, and not require any checking after the veset has passed, in the case of the pill the story is quite different. Let us assume that such a woman has established a veset to see 60 hours after she goes off the active pill, and now it has been 70 hours and she has not had her period yet. In such a case, it is hard to ignore the fact that she will see very soon, and thus many poskim here would agree that she cannot have sex until she has her period, or at least that she must do a bedikah, an internal check, before sex. Some poskim go even further, and would say that she must refrain from sex with her husband, not just starting from hour 60, but starting at the earliest time when she has bled in the past after going off her pill (so, even if she normally bleeds 60 hours later, if - from time to time - she bleeds 40 hours later, the couple has to stop having sex 40 hours after the active pill has stopped).
Because the standard periods to check for a woman who does not have a regular pattern are very complicated and often not so connected to the time when the period is actually expected, I advise re-teaching and re-emphasizing an oft-forgotten veset, the veset ha'guf. The veset ha'guf is a physical sensation that is linked with menstruation, i.e., menstrual cramps and related physical sensations. According to the mishna (63a) and gemara thereupon, this is the *primary* veset, so much so that it may not even require three times to establish it. However, to the best of my knowledge, this is almost completely ignored in chatan and kallah classes. This is quite fascinating, given that according to Shulkhan Arukh one needs to be concerned for this veset even if it occurs just once (189:19). Thus, if a woman once experienced such sensations before or during her period, she would have to refrain from sex when the same sensations occur the following month. As far as I know, this is never taught, and these sensations are regularly ignored.
One contemporary posek does address this. Rav Vozner, in his Shiurei Shevet HaLevi, directly addresses menstrual cramps and talks about establishing a veset when bleeding occurs during or a fixed time (or within a range of time) after menstrual cramping. In my opinion, we need to be focusing on this when teaching hilkhot niddah, as this is a veset that is very much linked to the real-world and to the expected time of menstruation. Thus, women who have such cramping and who can establish a veset ha'guf will both be freed from the complicated calendar-keeping that is the norm for a woman with a non-established veset, and will also be keeping a veset that is a genuine predictor of when she will have her period, and thus a genuine fulfillment of "And you shall separate the Children of Israel from their impurities."