Prolly fungus gnats. You usually get em from overwatering. Are you letting your soil dry out in between waterings? Either way, they can harm your plants' root system.
Get some sticky traps from the hardware store or garden store to catch adults, and chop some raw potatoes into cones. Sink the potatoes into the soil, but keep it where u can pull it out. the larvae will be attracted to the potatoes and u can just throw the pieces away every couple days.
You can also cover the top of your soil with sand. apparently in suffocates the juveniles and traps them so they can't take off. i've never used this method before myself.