It sounds what you’re describing is a mucocele, which is a tiny gland under the surface of your skin that fills up with liquid. Normally these glands are at work full time, creating the moisture in your mouth and keeping you comfortable. But when one of them gets blocked you may notice that an area will often swell up slightly, especially right before a meal.
Mucoceles are harmless and should be left alone, especially if they pop on their own or never get much larger than a BB pellet. We have thousands of salivary glands in our mouths – in our cheeks, on the insides of our lips, under our tongue and even on the roof of our mouth. They can sustain a small injury to the surface which may cause the gland to heal shut. If there’s nowhere for the saliva to drain it ends up filling the gland until it bursts. Salivary glands that are injured in this way may go up and down every day as they cycle through filling and bursting.
Most mucoceles require no treatment, but if you ever notice one that gets particularly large or that you get a large clear swelling in the center floor of your mouth, you may need to see a dentist or oral surgeon for treatment because you may have a ranula, which is a blockage of a major salivary gland under your tongue. Ranulas usually aren’t painful but they can make it difficult to swallow if they are allowed to continue swelling.