It all depends on the normal length of your cycle really as most people ovulate mid cycle. So for instance if you were on a 28 day cycle and you ovulated on day 14. Then the most likely days for getting pregnant would be having sex on days 9 to 15. (The sperm can live 6 days and the egg only 2)
If this is your cycle Then I am afraid as you count the first day of your period as day 1 you had sex on day 15 so the chances are very very high that you would get pregnant .
As it was only yesterday you can take a morning after pill up to 3 days after the time you had unprotected sex if you don't want to be pregnant .
The pill stops you ovulating (it would be too late for that part) But it also does something to the lining of your uterus to stop the egg implanting and as implantation doesn't occur until 7 to 10 days after fertilisation then you still have time to prevent this.
It is up to you to decide really what to do. Discuss it with your boyfriend.
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