Small whitish spot under tongue, back left. Very painful when moving the tongue around or swallowing.

Probable aphthous ul. An aphthous ulcer is shallow with a central whitish depression with red rim and usually occur on the mouth lining not attached to bony mouth structures. They typically last 7-10 days. Antibiotics or antiviral a don't help but they last less long with topical steroid creams. If it lasts longer than three weeks see ent.
Do not wait . The lesion could be benign. But the location is of utmost concern as that is a primary location for oral cancer. See a dentist asap for a diagnosis and possible biopsy. The good thing is that cancer may not be painful. Many dentists refer to oral surgeons for a second opinion.