Could be the change of seasons, maybe a certain alignment of planets, or anything else that you could be picking up on. While I don't believe in spirits and such, energies is something we do pick up on, and science is making vast progress in that field. Heck, it is a fact that full moon means more emergency calls. And then there is the well-known example of menstruation cycles following the phase of the moon and syncing in groups of women.
If it lasts, or severely bothers you, go talk to a professional. While the cause might not be found, it might make you feel more at ease.