I started a light period on cycle day 20. my average cycle length is between 28 to 34 days. flow is lighter than normal. could this be implantation?

One cannot tell yet: Wait. A positive pregnancy test can tell a woman that prior spotting was actually implantation spotting. It can be hard to tell if a woman is pregnant because some of the symptoms seen in early pregnancy can also happen in non-pregnant women. A urine pregnancy test gives a reliable result about 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a period.