Well if your on the pill I don't consider that being un protected sex. Don't know what the 'percentage' chance of getting pregnant is, but I wouldnt worry about it. I don't use any other kind of protection with my bf besides taking my birth control. And it being after your period that means your egg has already left your body (thats why you have your period in the first place because an egg drops and if no sperm is there to fertilize it breaks and thats when you get your period) (that was a short lame describtion of the female body but tried to make it quick hehe) so don't think you have anything to worry about :)