You may have given them rights to a particular story, but that doesn't mean they have the rights to everything you write on the topic, unless you signed a REALLY weird contract. As long as what you write now isn't too similar to the previous story (like, not copy pasted or closely paraphrased) and you don't reuse pictures you should be fine. You might want to see if you can find the original agreement with the magazine and what it actually said, and if you can't find it I'd suggest talking to someone with experience in copyright law.