I Am On Birth Control And I Had Intorcourse On

I am on birth control and I had intorcourse on December 12 and I was worried I may have gotten sperm inside me because no condom was used so on the 13th I took plan b. Ever since then I have been cramping and bloated here and there. On December 19 and today (20) I have experienced symotoms of having a period bad cramps and real bad bloating.. I want to know how long it lasts for and if it is normal.