salivary duct that occasionally becomes occluded (blocked). This is perhaps due to some minor trauma which periodically re-occurs. When the saliva duct does become occluded, the saliva gland will continue to produce saliva, temporary producing a "blister" like (fluid-filled space) appearance. The pressure of the fluid makes the lesion feel hard until it ruptures, releasing the fluid, and the cycle continues.
Nevertheless, I suggest having a physician or dentist check the lesion to make certain that there is no secondary condition which may be more serious which is the cause.