Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concerns..
As per your complain a painful knot under the chin can be most commonly an inflamed and swollen submental
lymph node and it can swell due to infection in oro-pharyngeal [head and neck] area and is commonly associated with infected teeth..
On the other hand a painful knot under the chin can also be due to infection in submental
salivary gland that is one of the major salivary gland..
As far as dental cleaning is concerned, it generally dos not seems to cause such symptoms and however seems unrelated to it..
I would suggest you to consult an Oral Physician and get her evaluated and he can do a proper evaluation and also if needed investigations like
x ray or MRI scan can be advised..
If infection is detected she can be given a course of antibiotics, anti-inflammatory painkiller like
Ibuprofen with regular follow up to access the progress of treatment..
As of now you can give her Ibuprofen or
Tylenol to relieve pain..
Do cool compresses over the lump to relieve inflammation..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.