Troubles with the IUD and scared Pregnacy.?

Okay, so I recently had the Mirena IUD inserted, (24th) the doctor told me it went well and sent me on my way..a tiny bit of the plastic tip was sticking out of the bottom of my cervix, but it went back inside, with advice from my mother not to worry about it.. On the 26th me and my boyfriend had .. Today, while checking my strings I noticed that I wasn't feeling threads but a piece of plastic, now I'm scared. I've looked every wheres but cannot find the answer. Does the IUD stop the pregnancy from happening right then, I heard that sperm can live inside the for up to 7 days, and now I'm wondering if if IUD would have stopped me from getting pregnant then, completely by destroying the sperm, not letting it get in or fertilize the egg, but would there be a chance that sperm maybe still be living in my and be able to get me pregnant now considering I think the IUD may be expelling it's self.