i’m confused

So i got an IUD about 3 months ago i have been sexually active with it as well. it’s one of the strongest forms of birth control and everything has been going fine. my period was a day late but it’s been like that ever since i’ve had the iud. this one i started bleeding but it was brown. i know that implantation bleeding is brown but it doesn’t last too terribly long. i had brown blood for the most part for 3-4 days and just wore a tampon. when i changed my tampon i notice that it had turned to regular blood, it was still light but i was bleeding for a few days after that. i’m unsure of what it could be but i don’t think i’m pregnant. help a girl out

i posted on another group but today i was at work and i haven’t bled for 2 days and i started bleeding again