What Causes Clear Fluid Filled Bumps On The Inside Of The Foot?

Hello, so I have these bumps on my foot that have been appearing for a few months now. It'll usually be one or two bums a few millimeters in diameter that don't hurt or itch. They can't be popped like you would a pimple, you need something sharp to open it and it oozes clear liquid. It only appears on the inside of my foot. I just want to make sure it's not anything serious. Any suggestions on what it could be?