If you had your period from the 16th-21st, you would have been on your placebo pills, correct? Even though you are not taking pills with hormones, you are still protected.
There have been countless times that my husband and I had sex during the time I was taking the placebos (though not on my period), and nothing has ever happened.
You 'should' be ok--but you won't know until your next period is due. You could be worrying yourself sick. Good luck.