I had unprotected sex two days after ending my periods and took an ipill on 3rd day after sex. i started bleeding after 6 days on taking i pill.. the bleeding is thick dark red colour.. should i be concerned?

Taking emergency contraception often causes irregular bleeding patterns and this is very consistent with that. It does not, however, indicate how well it worked. If your next period still comes on time then you should be OK, but do a pregnancy test if it does not. Emergency contraception is better than nothing but you should not rely on it. The sooner you take it after unprotected sex the better.