Had unprotected sex for the first time in 7 months while on implanon, no periods since i started the implanon and 2 days after unprotected sex i noticed light pink spotting when i wiped .. could i be pregnant??

Very very unlikely: While one shouldn't say never, it's very very unlikely to GET pregnant while on Implanon, assuming that one wasn't pregnant prior to procedure. Check out http://www.drugs.com/pro/implanon.html for more info. While obesity can adversely affect oral hormonal contraception eg birth control pills, there is not enough data on progesterone implants http://www.medscape.com/viewarticle/720094_3