After taking plan b

I took plan B on the 8th day after having my period . I had headache , dizziness nd feel like vomitting and starter cramping too. Then after 5 days of taking the pill, that is on 13 th day after my period, it started bleeding. It was not much on the 1 st day , but eventually, bleeding started increasing.. Have cramping nd back pain too.. Breast are sore and painfull.. How can i have period after 13 days after my last period date? I am worried is this the effect of plan B or anything else??