I’m very worried!

i had sex 3 weeks ago and last week i bleed one day and i was confused because i already had my period but the next day it stopped, but i had cramps, the next day i started spotting on and off and it was like that till Sunday (the one that just passed) my back was sore and i still had cramps then i started bleeding like a real period Monday and have been bleeding ever since, so i think it’s my real period. what’s going on? was it implantation bleeding?!