The patient is probably saying "I'm fine" when at the MD's office. So you have to sort out if the pain is real or the patient is being manipulative. In the meantime, give him/her over the counter meds. The verbal abuse suggests the patient may need an antidepressant as well. Ask the MD.
Can you locate a good pain management clinic? Pain can certainly lower the quality of life and make someone irritable. There are different things that a physician who knows how to treat pain can do -- oral medications, blocks, patches. There may be something that will work for the person you care for.