There were different sects then, yes, but "Orthodox Jews" did not exist then. It was Jewish, all of it, and you were either observant or not, but there weren't "denominations" like there are now.
So Ouiblette, there are some problems with your descriptions of the laws, they're not correct. But certain things we have as the law are very clearly specifically spelled out from that time via the Talmud, which tells us very specifically how various laws were observed, so we know that we follow these things the same way. Yes, there were no light switches to turn on back then, but the law works with the times to deal with the changes.
Which, by the way, is the difference between the Jewish denominations ... the Orthodox believe that the law does not change, and innovations (like, say electricity) are dealt with as per the law; and the Reform believe that the law does change with the times.
Anyway, The laws of nidda are dealt with very intensively in the Talmud (it has its own tractate, named Nidda, in the Talmud), so we know very clearly how it was observed back then. Which is not much different than the way it is observed now.
Edited by merpk - 12/11/10 at 11:23am