My period is lasting forever after i got the Nexplanon birth control!!!

So i got it 3 weeks ago the birth control that goes in my arm.. well since monday July18 and now i still have my period!!😢🥺😣😭 is this normal ?!!?? It wont go away and stop!!! The doctor didnt say that itll be longer he said that i wont probably have a period anymore but depending on me… idk !!! Im running out of pads already… I usually last on my period regular cycle 3 days exactly only. Not a whole weeek😫🙁am i ok? I gave birth to in March 2022 but idk if that matters.