Nexplanon inserted 7/29/16 no period since then. very light pink discharge only once today, is this a side effect?

Not uncommon at all: It is not unusual or uncommon that the progestins in Nexplanon cause you to bleed lightly, irregularly or not at all for months while providing you a very effective contraception. There is no cause for alarm. If you really feel that you need more reassurance, you may want to check your pregnancy test periodically or consult your gynecologist again.