Hello doc, i had intercourse with a girl and the next day she has her period. now she is pregnant and claims by me. is it possible?

Yes: Uncommon but possible. Pregnancy near menses is less common for most women but does occur. It would be more common with a woman who had irregular periods and menses is what happens in the womb, not in the Fallopian tubes which is where pregnancy starts.This may be a something where DNA confirmation during or after pregnancy will confirm or exclude you as the father.