If you become pregnant with an IUD in place, you need to have the IUD removed ASAP so I agree with PP, if you think you might be pregnant, keep testing every couple of days until you have a period.
Which kind of IUD do you have? The hormonal or the nonhormonal (copper) kind? I think missing a period is normal with the hormonal IUD but is NOT with the copper coil.
I would just keep testing, hun. Hopefully things will all work out for you.