Pregnant on IUD???

But I've been very sexually active for the last few months. And I think I might be having symptoms of pregnancy, because I'm experiencing a lot of the things I did the first time around and I can't explain them. 

I've had random nausea, which never happens to me normally. My moods have been all over the place and I've been crying over the smallest things, which is odd because my boyfriend complains about my lack of emotion and vulnerability sometimes. I've been having very scary, vivid nightmares (much like I did with my first pregnancy). And LOTS of fatigue, which hasn't been normal for me since I was pregnant last. Also, today I had some pretty bad cramps (which I've never experienced with mirena) and a lot of very sticky clear discharge. 

I have no clue what to do. I'm taking a pregnancy test in the morning, so I'll guess we'll see what happens. I'm just not sure how possible it is, or if I'm just getting myself all worked up over nothing.