I took emergency contraceptive pill n started bleeding after a few days n my period is after 20 days. this happened before also..will this harm me?

Periods/Plan B: Taking emergency contraception is quite likely to throw your periods off for a month - that's exactly how they work. If you had 5 days of bleeding shortly after the pill use, this sounds very normal and means you should not be or become pregnant. GIve it a couple of more weeks until your normal period would have been due and then if you have concern yet, take a home pregnancy test - but unlikely.