Hey welcome to FW! It is a truly amazing site. A lot of people worry about the same thing you asked about. Like Shari said, it's always possible, but you have your name attached and the date you submitted it so that all acts as proof.
The people here are amazing with their support and graciousness. If you do want feedback, yopu may want to come back here and leave a link to it. There are hundreds of new submissions each month so it can get easy to be missed. Remember if you don't get a lot of feedback or views it doesn't mean people didn't like it, but more that the ones who did view it didn't feel comfortable leaving a comment.
The best place here to get feedback is through the Writing Challenge. Those stories are definitely read by the most people. Also check out the critique circle.
The site is huge so try not to be overwhelmed, but jump in and join the discussions or ask any more questions you might have.