I started my period a week early. It was very light & lasted only 3 days. I should've started my period by now. Was it implantation or am I pregnant?

Possible pregnancy. If a period comes at the wrong time and is lighter than usual then pregnancy is possible, so check a pregnancy test. Irregular bleeding can also occur if the hormones are off, and most of the time the cycle will go back to normal. So if the pregnancy test is negative it's fine to see what happens next month.