Cramps & brown spotting for 3 days. this was 6-8 days after i stopped bc & had unprotected sex the 1st 3 days after stopping bc. is this implantation?

No: but could be a sign of pelvic infection--any vag Discharge or pain with intercoarse? Unless you had an IUD for birth controll in which case it may be from removal of the IUD---and should go away

Not likely: implantation is usually 8 weeks or so, so no. Lots of questions, where in your cycle did you stop birth control, likely screwed up hormones? get back on birth control or no birth control and let your body get used to something