Yes it is possible to get a false negative and still be pregnant. False negative are very common and having a IUD doesn't effect the test results either way.
You can either keep testing once a week with 1st morning urine until you either get a positive result or a normal for you period or see a Dr. for a blood test.
Best of luck.
Me:35, DH:45, DD:17, DD:17, DD:16, DS:13, DD:11, DS:9 and 2 cats.