I was on the same medication and was given a FFN every 2 weeks for 6 weeks. It came back negative all but once. I delivered early but was induced. From my understanding the test only tells you if you have a chance of delivering early within a two week period but it doesn't tell you that you won't. It's very ambiguous. At the same time I was in and out of the hospital with contractions, bedrest, taking that test, high blood pressure, etc. my girlfriend was too. She was also given that test. It was positive and she didn't deliver early. I think it's just a precautionary test for your ob's reassurance. That way, they can watch closely your signs of preterm labor.
I wouldn't stress out about it.
Good luck - take it easy - and enjoy!