Period is almost 2 weeks late. Now I am bleeding lightly. Had sex 1 month ago. Normal menstrual cramps and backaches. Implantation bleeding or period?

Breakthrough. It's probably breakthrough bleeding, can't say for sure 'why' over the internet. 'implantation bleeding' is all the rage online now, but it's over-rated in importance. It's not consistent and is usually not 'implantation' but breakthrough bleeding. A pregnancy test in a few days will confirm one way or another.