Implantation bleeding or short period??

Hi all! So I got my Mirena taken out April 27th, a week after my last period. We've been having unprotected sex but I have no idea when I'm ovulating because I hadn't had a period since October. I was due to start AF May 16th. Well for about a day and a half May 15-16 I spotted. Nothing heavy and pinkish, brownish in color. Could this have been implantation bleeding or just a really weird period??