PLAN B

I got the mirena out 7 days ago.  I immediately started taking birth control.  I asked the doctor when I can have sex again and she said in 7 days after starting birth control.  So we did tonight but I thought about it after and how can it be 7 days????  I mean I was always told a month. So why did the doctor tell me 7 days.  Should I take plan b just in case ?   We have a 9 month old and we are definitely NOT trying to get pregnant.