What Causes A Clear, Fluid-filled Bubble Appearing And Disappearing On The Roof Of My Mouth For The Past 5-6 Weeks?

I have had this clear bubble come and go for about 5-6 weeks now on the back of my mouth below my arch of the roof of my mouth. It slowly appears and disappears practically over night and can be gone for a few days at a time. It looks clear like a fluid filled bubble.