My boyfriend ejaculated inside me all week but i started my period right after. could i still get pregnant?

Pregnancy has been documented from unprotected sex on any day of the cycle including during menses. Any unprotected exposure can get you pregnant, even on days that are called "safe". It is uncommon and would be more likely if you had irregular cycles. However, the risk remains and next month you ay find out.

If you got your usual period, after sex, you are not pregnant. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex : https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines

Since you started your period, chances are you are not pregnant. With risky sex habits, next time you could become pregnant. To avoid pregnancy practice safe, use a condom or other methods of birth control.