Hello Iris
We are sometimes born with restrictive labial or lingual frenum. Do you remember having had a fall when you were a child and 'tearing' this labial frenum? This residual nodule is benign and is probably a remnant of that torn connective tissue. It can be easily corrected with dental laser therapy.
In my clinics, we use the Waterlase iPlus laser to help patients with frenula releases.
You may want to check under the tongue for a tight or restrictive frenum too. Studies from Stanford University have shown that short lingual frenum unattended at birth may cause obstructive sleep apnea at later age.
In some cases, it is upper airway resistance syndrome. Do pay attention to sleep quality and breathing during sleep. It is crucial to overall health.