The CONSTANT single fly, what does it mean??

I've read that flies can be representing of negative, or even evil energy. There's been this one singular, tiny fly in my room. I've actually killed it a couple times, but for some reason it keeps coming back?? I never see more than one at a time, and the time that it comes back around is nearly instant, or at least a day.

Some things to consider is yes I do sometimes keep food in my room, but never long enough to become an issue, I don't even have ants. I also have plants in my room? So those factors could also play a part, I'm just genuinely not sure.