Nauseous all the time

Megan
Hi, I'm 23, recently got the Mirena IUD in August and for about two weeks now I have nausea pretty much all day, every day. And intermittent cramping even with no bleeding. I'm not vomiting at all. 
Yes, I have had unprotected sex but I am also having a period now...
What do you think this could be? What could help? To make matters worse, I cannot feel the strings (my dr. did say he cut them short and that I don't need to check though, but the Internet gives different advice). I'm at my wit's end. Thanks so much. 
Update: took a test and it was negative.