You are getting "hmmms" from your dentist and your PCP because they do NOT know and are not going to attempt to guess which is prudent on their part.
I think an Oral Surgeon would be more appropriate to follow up with vs ENT. Oral Surgeons are more in tune with dealing with your situation; assessing growths/abnormalities in the oral cavity. I am confident the Oral Surgeon will be able to give you a definite answer as to what this is. He/she will assess this "lump" and probably biopsy it.
The lump behind the tongue should be either hypertrophied lingual tonsils or the ectopic thyroid gland (also called Lingual thyroid). It is because of the chronic irritation in this site could have caused the enlargement of the lingual tonsils and hence the glands in the neck, or lingual thyroid was noticed accidentally during your dental checkup
Rather than speculating things, please do consult your ENT/Oral Specialist. Any more doubts you are welcome.
Speaking of Thyroid, I am currently now tking medicine for my Thyroids. Their called LEVOTHROXINE 0.075 MG. This issue with my thyroid was discovered like 6-9 months ago. I will keep you updated though when visiting the ENT wich is this Tuesday the 13th. Thanks again to you and everyone responding to my questions.