It could be most anything, from an overcrowded pot to too much watering. Try and think back the past few weeks and recall of any fertlizers or sprays were used on it. If its indoors, it may need more sunlight. If its bugs, you may see tiny black gnats that are called fungus gnats and these can destroy plants, as well as infect every single plant indoors our outside. One way to know if its the gnats is to use yellow sticky traps and set one horizontally on the edge of the pot. Wait several hours, and if you see tiny flies stuck there, you have the gnats. My garden suffers from these as well, so I use either a Bt product to kill the maggots they produce (feed on the roots!), and also beneficial nematodes. It can also come from any soil you use as well as reused pots that are not sterilzed, and could even be in indoor plants right now. Oregano is one herb that doesn't like fertilizers and too much water. If its gnats, they can fly throughout the whole house and infest every single indoor plant too!