Why is there a hole in my composite filling?

I had a composite filling on my molar tooth in the top row a couple weeks ago. But yesterday, I found my tongue lingering on that tooth, so I checked it out in the mirror, and it appears that a hole has appeared in my filling! It wasn't on the natural part of the tooth, but the filling. It was on the front, and it's quite a big hole, if I'm going to be honest with you, and I'm not very sure, but it seems to have appeared overnight or something. I flick my tongue in my mouth a lot, and I didn't feel anything 'unusual' in my teeth before. It might be that it already existed before, but I didn't notice, though. It doesn't hurt either, but I am still quite concerned. Did the filling break down or do I have a cavity again? I brush very carefully 2 times a day, for more than two minutes. I use mouthwash two hours after brushing at night and floss once a day. I use an electrical toothbrush. If it is a cavity, am I doing anything wrong with my oral hygiene routine? I'm very sorry about the blurry picture! It's not clear, but I hope you can see that there's a hole in the right molar tooth when it's compared to the one at the left side. There should be something covering the front up in the right molar tooth, but the whole chunk of that is gone. I'm also very sorry for the long text, but thank you to the people who read this to the end and to the answers in advance, too.(And if you are wondering why I have such little teeth in the picture, I have a genetic condition where my teeth doesn't grow back after it's baby version fell.)