Ok, first of all it's spelled "anal". He put his penis in your anus, and that's the only place he put it? And he ejaculated inside of your anus, right?
Well as I said before your chances of being pregnant are very very low, so don't worry about it, as long as he was not inside of your vagina. You should use condoms, and then you won't have to worry! Get on some birth control or something before having more sex!
No, becoming pregnant is not going to stop you period, because it's near impossible to become pregnant when you're on your period, especially if you're a few days into it.