Welcome in H.C.M.
I have gone through your query regarding stiffness
under your tongue, though your M.R.I. is normal.
Your clinical story direct us to think over about inlammatory diseases of floor of mouth which may be expected like-
1- Submandibular sialoadenitis.
2- Submandibular cellulitis
4- Submandibular sialolithisis (stone ).
All above may be ruled out by a thorough examination by ENT doctor and by doing-
of submandibular region.
b- X-Ray submandibular region in Occulusive view.
Probably above answers your query.