First, I just want to say, don't be scared! Or be scared, but know that you're not alone in fear of an unplanned pregnancy. There are lots of other women who go through this, and that's why there are so many resources to help you pick the path that works for you IF this is an unplanned pregnancy. You've got this, no matter what!
Now, onto logistics - yes, it's possible to get pregnant on an IUD, even if those chances are small. It's also important to find out as soon as possible if you are since there's a risk of ectopic pregnancy when you have an IUD inserted.
So, here's what you need to do - get the pregnancy test. It won't be a waste of money because your peace of mind is worth more than the few dollars the test will cost.
If the test comes back negative and you're still experiencing these uncomfortable symptoms, you need to let your doctor know, since it could be a sign that you're just having an issue with your birth control.
If the test comes back positive, give yourself a minute to gather your thoughts and know that no matter what you decide from there, trust yourself.