For Tamiflu to work on the flu and stop the flu you need to take Tamiflu 48 hours before the onset of the flu.
Once you take Tamiflu then you should no longer be contagious after 48 hours once the Tamiflu has been allowed to work on the flu virus.
If you don't take the Tamiflu within 48 hours of the flu then you'll remain contagious for at least 7 days after the onset of the flu virus.
Without Tamiflu you're contagious from day one of the flu.