Is this possible

You will not get pregnant from oral sex by itself....your digestive tract and reproductive tract do not mix. Only possible way would be if he was inside your vagina first then you had oral sex as there can be sperm in the semen that is present during arousal, or if you had oral sex....he ejaculated, and then put his penis in your vagina right afterwards....inwhich case there could still be semen and sperm present. Odds are slim in both cases, but it is possible.
If you are normally very regular with your cycle, it could be your period is just off this cycle.