IUD.. Got my period after sex??

Soo, I got an IUD at the beginning of January. Today was the first time i had sex since getting the IUD. Ever since i got it i have been spotting everyday. It has been dark brown blood. BUT today.. About an hour after we had sex i started bleeding regular red period blood.. Enough for me to put in a tampon. Can someone tell me whyyy this is happening?? I'm super confused.. Slightly worried