Brief Answer:
different possible causes

Detailed Answer:
HI, thanks for using healthcare magic

Pain at a joint can be due to changes in the joint itself or related to surrounding muscles and ligaments.

The diagnosis would depend on : (1) age of the person (eg for your mum- osteoarthritis is the most common reason for hip or knee pain in older persons) ,
(2)type of pain,
(3)when it occurs (eg is it present on waking then improves somewhat or worsens with movement throughout the day)
(4)what worsens it
(5)history of trauma even excess physical activity or exertion
(6)any additional symptoms eg in some types of arthritis eg RA, there are general symptoms such as fatigue or stiffness
With osteoarthritis there is morning stiffness lasting less than 30 mins and the pain tends to worsen as the day progresses.
With inflammatory causes of arthritis (RA, lupus etc)- pain occurs with or without movement but can decrease somewhat as the person uses the joint

The main causes of hip pain would be: (1)trauma
(2)arthritis and there are different types: osteoarthritis, rheumatoid arthritis, lupus related, psoriatic arthritis (associated with the skin condition psoriasis), septic arthritis (due to infection of the joint- normally there would be a fever), gout, lyme disease
(3)injury of the surrounding muscles, bursa and ligaments can also cause pain

To find the definite cause of pain in any joint in any person, the joint itself would need to be examined along with a general examination.
Imaging of the joint by at least an xray is also recommended.

Some form of imaging is needed for a definitive answer

If this is not an option at this time then treatment would consist of rest of the affected joint, warm compress if possible 2 to 3 times daily and the use of pain medication such as ibuprofen, naproxen, celebrex etc
In some persons, depending on the cause, physiotherapy would also help as well

I hope this helps, feel free to ask any other questions

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