Unless you started the pack on the first day of your period, it is standard practice to wait one week before having sex without a back up method. Since you had sex on day six, it is possible that you became pregnant. Plan B is an option of you have unprotected sex again. (I don't recommend taking extra tablets. If you do that, you mess up your pack and run the risk of not taking the birth control as directed. This could lead to an unexpected pregnancy.) However, I hope that this close call- if you turn out to not be pregnant- would be sufficient to make sure you don't have unprotected sex again until you are ready to have a baby.

I did start the pack on day 1 of my period. Does it mean I'm safe? (Please, please, say yes, I can't stop thinking about it)....I've read a lot of info about birth control and unprotected sex and some are contradicting so I'm not quite sure what to believe.

Ideally, you should start taking Marvelon on day one of your menstrual cycle (the first day of your period). This will protect you from pregnancy immediately and you won't need to use any additional methods of contraception.