I had unprotected sex saturday night and got my period the following night. do i still have a chance of having become pregnant? should i take plan b?

Low chance, but..: A healthy woman of reproductive age can become pregnant any time during her cycle, highest chance during/around ovulation and lower during active menstrual flow. It is very difficult to know exactly when ovulation occurs, thus the risk of pregnancy if not using protection. Although your chance of pregnancy is low, planb can reduce that chance nuch further. Be safe. Use protection. Good luck.

Not likely: Having your period means the uterus was expelling its lining. It takes two weeks or so to replace it. Therefore you can't be pregnant. Please start using birth control and condoms to have a healthy sex life.