After 6 days of taking plan b pill, i got a heavy flow period for only 2 days & now only brownish discharge for 2 days. is it implantation bleeding?

Unlikely: It depends on how soon after your unprotected intercourse, that you took plan b. The sooner you took it, the more likely that you prevented ovulation. Implantation bleeding is a bit of a misnomer. It sounds like your bleeding pattern may be off for awhile. If it's been at least 10 days since unprotected intercourse, can take a home preg test. If neg, repeat 7 days. Consider birth control also.