Hi, i've had my period three weeks ago on april 5th, 2016. for the past few days i've been bleeding lightly, and today it came out a lot ?

Your cycle is in: flux right now and will probably reset it self again in 1-2 months if you allow it to. Bleeding irregularly is not uncommon in your age group. You need to watch out for vaginal discharge and/or pain/itching, those symptoms suggesting infection. If you have lots of bleeding for 2-3 weeks on end then perhaps you will need to shock your hormones into resetting. A healthcare provider can help.