What Causes Nausea Feeling During Intercourse After Inserting Mirena?

I just got the Mirena IUD a couple days ago. Up until that time, me and my boyfriend were VERY careful, and I'm not pregnant obviously. Now, just the other day, we had sex and now that I have the Mirena we feel it's safe to not use a condom. But, I got nauseous after he came inside of me. Is it his sperm or just the mirena acting up.