Lighter than normal period

I had my period on 4/6-4/10 which is the normal because my period usually only lasts 4 days but it was sooooo light. I went to my gyno and she did a quantitative pregnancy test which came back negative my hCG level was less than 2. So she decided to wait a week and do it again my levels still less than 2 so I know I'm not pregnant. But what can cause light periods? I did recently start some new medications and been a little stressed over little things but nothing major. I usually have a fairly heavy period with cramping and I just don't understand why I barely bled or cramped this month.