Very possibly fungus gnats which breed readily in potting medium if kept on the moist side. The tiny larvae, which live in the soil, can become a problem for plants if their population is too high. They will feed on the roots.

If you mulch your plants with aquarium gravel, the adults will not be able to lay their eggs in the potting soil. Allow your plants to dry out a bit between waterings, if possible. Next year, plan on using a medium with improved porosity, or fix the problem yourself by adding a large percentage of perlite to the mix before potting up.

Thank you for your help!