Thanks for your question, and so sorry to hear about such a serious family history of cancer. It is hard to say for certain what that lump may be, but you understand better than most people that you should discuss this with a doctor
to make sure that it is not something serious. Most of the common reasons for a lump under the jaw do not also contribute to headaches, and so it could be that you are having something else that is causing both the lump and the headaches. For example, if you were having an upper respiratory tract infection, you may have swollen lymph nodes as part of your infection, and some of these could be under your jaw. You may also have headaches as part of your symptoms. That is just one example, however, and you need to speak with a doctor. Also, it may be worth while for you to meet with a geneticist
to see if there is any specific cancer risks that you may have. More importantly, a geneticist may be able to help make some recommendations about different things that you could do or could avoid to help to decrease your risk of also getting cancer like so many of your family members have done. Please speak with a doctor.