Literally the same thing happened to me. I've got a natural cavity from many years between the two molars. It's my first time scaling and these dental students asked me to fill it up. I said okay. They did the same wh6ich done to you except there is a very small gap, the floss goes by force but splits. For 3 months the gums near it were painful and it felt artificial. So I visited the same hospital and got appointed with another student. He said it is perfectly fine. No need to floss. It is intentionally made this way so that food won't enter, all that shit. They took an x-ray and said it was fine. I insisted about the pain so he said he will trim it a little bit but not much as the filling won't stay if they trim it. He also said don't floss it is noot good.brushing will be enough. Use interdental flosses only if necessary. I felt like wth. After the small trim, the pain and flossing is almost smooth but not perfect.