It does sound like implantation & no contraception is fool proof! Like Stacey said-have you missed a pill or been sick? If yes your chances are higher.
Wait until until your period is due & if it doesn't show take a test.
if you wernt on the pill I would say that it could maybe be a sign of implantaion, but unless you've been abit wonky with taking the pill or your on the "mini"pill I doubt that its pregnancy or ovulation, but as always stranger things have been known to happen so you cant rule out anything, if your period doesnt arrive on time or is in any way different for you then I would suggest taking a test