I have a small cyst on the floor of my gums.it never completely goes away but keeps shrinking then filling up again.

Small cyst: It sounds like you have an infected tooth. See your dentist asap for treatment.

Not sure what you: Mean by "floor of my gums". If you mean floor of your mouth, as in under your tongue, then this could be a ranula. A ranula is caused by saliva from a blocked salivary gland backing up under the skin. They often can drain and then refill. Regardless of what this is, you need to have it evaluated in person by a dentist so they can give you proper advice.