Hello Jennifer, As Brian said no one on this board can give you a difinitive diagnosis. It is best to be seen by a dentist first off for a oral exam.
In my experience, rdh for 16 yrs, the "knot" on the floor of your mouth could possible be a simple as a blocked salivary gland or a cyst. It should be examined by a oral care specialist. If insurance coverage is a factor many teaching facilities will offer exam's for reduced fee's. If there is a dental school in your area that could be an option.
I hope this helps, Linda