Why do my videos get blurry after one day of downloading them?

Hello! I have this major problem where i download a video that i edited in 4k from CapCut and the day that i download it in, the quality is the same, very high. But the following day i notice that my quality has dropped to like 480p. Why is that happening?And also, even without downloading from CapCut, when taking a screenshot it looks good then the next day the quality is awful. Please help, I am a Content Creator and I need my phone a lot!