Whats this thing on top my teeth on my gum?!

Its pink (obviously because its on my gum), ontop my tooth.. It doesn't hurt unless i press down on it. Sometimes it whiteish and looks full of fluid, So i pop it and it comes back 5 seconds after bigger. I put baking soda, brush my teeth, gargle with salt water, and use Listerine on it almost every night then it goes away.. but whenever i forget to brush my teeth one night it'll come back, is it bacteria ?