It might be that the Wharton's duct leading to your submandibular salivary gland is blocked. This could be the opening of the duct. I have one under my tongue as well and that is what the doctor says it is. It is curable.
You are not alone being worried off and on about cancer. Health anxiety is real! :-) Has the bump gone away? You posted this a bit ago so an update is helpful. Bumps under the tongue usually go away on their own and are nothing serious. An injury, a virus, could be a little oral mucous cyst. Tumors will often have other things associated with them. Is this gone now? Here's something to copy and paste into your browser to read about it. https://www.medicalnewstoday.com/articles/bump-under-tongue#diagnosis