This may not be because of your breast cancer
and treatment but could be coincidental. If the pain and crunching you are felling in on the thumb side of your wrist then the diagnosis could potential be a tenosynovitis
where the tendons to the thumb become inflamed a the level of the wrist. DeQuervains Tenosynovitis as it is called can be helped with anti-inflammatory medication, a wrist splint, local anaesthetic
injections and occasionally surgery. I would seek your family Dr opinion to the diagnosis and further management.