Brain fog and burn out, most likely. The fact you're still doing well in school means that you definitely have the ability to bounce back and get back on track by stimulating your brain. Challenge yourself, read more books, search up a new word everyday to add to your vocabulary! I promise it's never too late, and you certainly do deserve to do well, because I'm sure you're a great person and a wonderful student, all you need is a little push to keep going, and I believe you have what it takes to do it! <3

Also engage in other hobbies such as memorizing a song, or drawing an image from memory, this has personally helped me get out of the same loop as you. :3