What Causes Bulging Of The Labia During Masturbation?

Hi, may I answer your health queries right now ? Please type y.hi i had a barthelin cyst 3yrs ago.and that time i did a surgery to drain it.bt now a days whenever i do masterbate the opening of labia bulges.if i stopped masterbate atleast 2/3days,it disappears.im worried.pls help me.