it is possible to get pregnant while on the pill, even if you take it every day on time. Best bet is to use a condom while still using the pill. You can not always go by symptoms because they are so common in other cases. Good luck!
if you are on the pill right now and you need information, I'd say the best thing to do is to read the insert that comes with your pills, and most pills have a website to help you with your prescription.
the only way to know if you are pregnant, is to wait it out, and do an hpt the day of a missed period. assuming you miss it.
symptoms for pregnancy are pretty much the same as your pms symptoms so it is hard to differentiate between the two.