I take plan b because l had unprotected sex with my bf and its my 11th days since i have my period and it still coming strong is it a serious problem?

No: The Progesterone in the Plan B can cause breakthrough bleeding, interrupted period and thinning of the lining of the uterus, such that a woman has more extended bleeding time. It can take 3-4 weeks for cycle to normalize, depending on when in cycle Plan B was taken. So long as you've taken pregnancy test and it's negative, just wait it out, you'll be fine.