Plan B can have some lasting side effects on your cycle. If everything doesn't go back to normal around three months after taking the pill then you should speak with a doctor.
After taking the morning after pill you can't expect your period at any particular time. It could come early, late, spotting in between, really heavy, really light... and it will be completely normal. The fact that you've had at least spotting every month so far sounds normal and it wouldn't surprise me if the extremely light spotting WAS your period for the month.
Eventually, things should even themselves back out and your periods should go back to normal. You can keep testing for pregnancy, but this sounds normal considering you took plan b.