I'm on day 1 of my period and i have a 40 day, regular cycle. my boyfriend and i just had sex and he ejaculated inside of me. chance of pregnancy?

Low but not Nil: There is always a chance of getting pregnant. I would suggest to contact your doctor to get the morning after pill. Please be aware that the sooner you take it the better it works, and it is most effective in preventing pregnancy up to 72 hours after intercourse.

Possible: certainly any unprotected intercourse with penetration and ejaculation can lead to pregnancy. I suggest a home pregnancy test 5-7 days after the sexual intercourse. I also suggest a visit to your GYN for definitive BC. In the interim use condoms PLUS spermicidal foam, gel. vaginal insets etc.