Hey it could happen, My birthday is oct 3rd, and I was on my period, and I found out that my son was concieved around the 7th of oct ( this was in 2006) so I don't know for sure if it could happen the day after your period but it sure happend 4 days after mine!!
I am a firm believer that you can get pregnant at any time. Your body may have an internal clock but it doesn't run like clockwork. There are perchances and happenstances that are always going on inside the miracle that is the human body. When I went back and calculated my conception date I was no where near when you are supposed to be most fertile.