Usually day 3 or 4 is the heaviest of your cycle, especially if you have 7 day periods. Personally, I find by day 4 I am getting lighter.
Periods change based on stress, when you last had sex (sex makes them lighter and shorter), diet, hormones and sometimes even vitamins.
So long as your period isn't becoming excessivly painful or you have no period or a very heavy period, I wouldn't worry about it
