Why I have lighter period than usual for 6 days?
The female cycle is a basically a biologic time piece. Works well enough for the human race to make this far but it is not as accurate or reliable as a modern mechanical time piece or even a time piece of a century ago.
Even with an impeccable every 28 day menstrual history there will be times that things will be off in the timing or amount of bleeding.
You don't give your age or menstrual history so speculation as to "why" would be a useless exercise.
Having said all of that, I suggest you make sure pregnancy is not the issue. A pregnancy test 3 weeks from your last coitus would do it. Then if your symptoms are very unusual as compared to your menstrual history make a routine appointment with your doctor.
If the symptoms are just a variation of previous periods you can wait to see if it is a recurring problem.