What Causes A Light Menstrual Period And Fatigue?

im 20 years old..me n my boyfriend had sex last week monday and my period came on that tuesday morning but it was very light i had no cramps and it only lasted for 3days but it always lasts for 5-6days and it always comes with excrutiating cramps in my lower back..thats one part of the situation the other is that since yesterday i have been sleeping alot..i find it very hard to sleep in the day but yesterday and today i feel like i haven't slept but i go to bed usually around 9 or 10 n im up every morning after 7:30/8am. i have never been like this before and i dont know why this is happening to me now